A button found in many web browsers that returns to the previous page in browsing history. (The chances are that if your web browser doesn't have one, you should get a better browser, or get an appointment with a psychiatrist in advance...)
Back button is an important part of web usage. According to Jakob Nielsen, it's the second-most used thing after links themselves. Some sites try to break its functionality - this is often just plain annoying.
Leave the Back button functionality for the back button and do normal links just like everyone else...