"somebody" and "someone" are pretty much interchangeable http://grammarist.com/usage/somebody-someone/ gives some more details. They basically say that it depends on the sentence and personal preference. I'm not sure what your question is regards 'nobody' and 'anybody', as they are very different.

Rewel, Thank you very much! :)

Gary answered the question about "somebody" and "someone" well, so I'll refer you to his comment for that! "Anybody" is very similar to "somebody," and the two are usually interchangeable when speaking. If you want to get very technical, "anybody" tends to be more general while "somebody" usually implies that the speaker or subject is being a little more specific or has slightly more familiarity with the person or group being referred to as "somebody." "Nobody" is the opposite of "everybody" - it is used the same way " no one" is to exclude all people when talking about something (for example, "Nobody knows where he went."). As Gary mentioned, "anybody" and "nobody" are entirely different words, as "anybody" refers to and includes at least one general person in whatever is being referred to ("Anybody can eat two sandwiches easily!" ), while "nobody" looks to exclude any person involved in whatever is being referred to ("Nobody can eat that much food. It just is not possible.").