Question:
I'm originally from Somalia and I get asked by many people about my identity.  Especially why we Somalians and Ethiopians look slightly different than other Africans.  Sadly I do not know much other than the fact that I am African and of the black race.  Perhaps some information on the history of these ethnicities will help.

Answer:
The question you ask actually involves some very complex factors.  Initially what I will do is give some background context for the factors involved.  Forgive me if at first some of these comments seem too detailed or far from your question.

I have visited Ethiopia on several occasions also, as well as Eritrea, and have studied extensively the backgrounds of the Amhara and Tigrinya and related peoples.  Perhaps my articles on their ethnicity will be helpful:
Tigre, Tigray, Tigrinya - Ethnicities, Languages and Politics
Tigray-Tigrinya
The Amhara

Race and Skin Color
Here is a basic principle that underlies such questions.  Centuries of systematic investigation and analysis of information about the world's peoples, informs us that there is no consistent relationship of skin color to "race."  Further, the concept of "race" is very vague and is not a technical category with a standard definition.  Further, geneticists have discovered that there are different sets of genes controlling skin color that are triggered for other reasons in different populations.

Race
There are some general informal categories, sometimes called "race," that help us classify various peoples of the world, but they are not strictly defined, and are used differently by different writers.  The terms and categories used do not relate clearly to definite physical types.

This is one reason you will sometimes hear it said that there is only one race, the Human Race.  All humans now living on Earth apparently share the same full gene pool.  There is no simple relationship of physical features (physiognomy) or genetic structure to any clear groupings of peoples.  We can of course perceive some general patterns, and these are the basis of our broad distinctions between various individuals and population groups.

In the past physical features has been one basis on which some have tried to categorize groups of humans.  A major problem with this and some other approaches is that the categories are used for discriminatory and exploitative or repressive purposes.

Common Genetic Stream
Findings in Archaeology and Genetic studies continue to indicate Africa as the point of origin of the human race. However, recent genetic studies have shown that the human group with oldest sequence of genes is another African group of atypical appearance. The oldest genetic stream of humanity is found among the "Bushmen" of Southern and Central Africa, more properly known as the San (or Khoi-San) people.  These genetic studies include a comparative study of select peoples of all continents and physical types.

Since the mapping of the human genome in recent years, DNA samples from around the world have been compared.  This has enabled specialists to establish a clear geographical chain of origin, descent and migration for human populations worldwide.  Spencer Wells has written up the historical conclusions from this comparative DNA study, which reads like an exciting novel:
The Journey of Man:  A Genetic Odyssey.  Read my review essay on this book and its subject.

A direct line of descent can be discerned in a sequence of chronological mutations discovered on the Y chromosome.  Sequential patterns of change are similarly discernible on the mitochondrial DNA of the cell nucleus (from the mother's heritage).  The physical differences associated with some of these gene changes over thousands of years are expressed as differences in appearance and body type between different human populations.  Sometimes these observed differences are the basis of a naïve categorization referred to as "race."  Yet these superficial differences occur among humans whose DNA is 99.6% identical! [Francis S. Collins, The Language of Life (NY: Harper, 2010) p 146]

Visual Continuum
Among the broad human "races," there exists a continuum of physical features and skin color.  There are no clear-cut categories by either skin color, hair or eye color, head shape or size, height, body size, leg length or other feature.  Only general groupings and greater tendencies in various groups.

Traditional terms (developed in Europe) for identifying broad human groupings, like Caucasian, Negroid and Mongoloid, are too general and inadequate to account for the variety we find among humans.  Findings since those early prescientific classifications help us appreciate and enjoy the rich variety we share in our human family.

Skin colour is not directly tied to any certain facial features.  Skin colour distinctions, like "white" and "black," are also very general and inexact (where is the border between very white and moderately white, or more white than the neighbour, etc) and are useful only for relative distinctions.  Terms vary from region to region and country to country, depending on the features in focus locally.  There are black peoples in Southern India and the Indian Ocean Islands with so-called "Caucasian" features.  In general this group/cluster of people is called Dravidian.  Examples of Dravidian peoples are groups in India speaking languages like Tamil, Telugu and Kannada.

Peoples called Melanesian in the South Pacific are also black, and have features somewhat similar to Negroid Africans, but they don't really look like Africans.  The peoples of Papua New Guinea and Irian Jaya, for instance, are Melanesian.  Likewise in this group are those called Australian Aborigines ("Originals").  Indians at the southern tip of South America also have "robust" features, but have light-coloured skin.

Thus there are peoples who are black that do not fit into the classic group of physical features traditionally referred to as "Negroid."  The geography-based designation "African" is generally used in popular speech to refer to the broad majority of black peoples of Africa who generally fit the popular traditional classification of physical features called Negroid.

Ethnicities in Northeastern Africa
Now when we come to the Horn of Africa, there are basically three groupings of peoples involved in the ethnic mix of Ethiopians and Somalis:
Cushites, Semites and Bantu or Nilo-Saharan.  The best way to group these various peoples is by language, since languages can be more objectively organized and the relationship between various languages has been well-established over the last two centuries.

Each of these broad groups also includes complex sub-groups that complicate the language and ethnic picture in North-Eastern Africa.  The primary way groups of ethnicities in the Horn of Africa seem to relate is by language, more than by physical type.

My articles about the Beja and Oromo peoples in the region also provides some historical information and discusses how different ethnicities intermarry and how new peoples emerge.  The Beja are a group of basically Cushite peoples who have mixed in various ways with Semitic peoples, the Sabeans and Arabs, as well as with other types of Cushites.

The term Oromo refers to a large grouping of languages and peoples in the Horn of Africa and southward deep into Kenya.  These languages are Cushitic and are related distantly to the Somali/Maay group.  The Oromo peoples are generally very dark, like the Somalis, but are shorter and have more "African" (Negroid) features, perhaps due to more intermarrying with Bantu or Nilotic groups.
The Oromo: What Factors Make a People Group Distinct?

A related Cushite group is the Afar, on the Red Sea coast of Ethiopia and Eritrea.  They seem to be generally a lighter color, with skin tones that may be perceived as brown.  Their language is in the same broad group as the Oromo and Somali families.

Mythical and historical elements merge in the oral traditions of the region. For more historical and cultural information, see my article about the Afar:
Afar, Ophir and the Mists of History

Migrations and Ethnic Overlays
Among the peoples of the Horn of Africa, like every other place in the world, various groups of humans have migrated in and out over the millennia, and intermarried in various ways.  They have exchanged culture and language.  Some Cushite peoples have migrated west towards the center of the Africa continent, like the Tutsi in the Lakes Area, according to some analyses.

Meanwhile Nilotic peoples from the North have migrated south to mix with Cushite and Bantu peoples.  You can read about some of these in the following article:
Tutsi, Hutu and Hima - Cultural Background in Rwanda

The Bantu peoples (named for their related languages) in Eastern Africa and Somalia seem to have migrated from the center of the continent from around the time of Christ.  Historical linguistics leads to the conclusion that they had made it into what is now Southern Somalia perhaps by 200 AD.  Related to this migration are the "Somali Bantu," Gosha, Mushungulu and related groups in the inland Horn, as well as the Barawa (Brava) of the Somali coast and related Swahili groups on the coast of East Africa from Kenya to Mozambique.

Incidentally, the Swahili groups tend to be lighter skinned due to their origin from the intermarrying of Arab men with African (Bantu) women along the east coast of Africa.  See more in my article on the Swahili peoples.