What is meant by "Evangelical"? What denominations are included in this grouping and why?

It appears that the term "evangelical" is one of the most used category of Christians referred to today, but also the least well-defined, imho. On this website, I found an article talking about how the term is used opposite the term "fundamentalist", and sometimes in concert with that same word. It has also been used opposite the term "liberal", implying that one can't be liberal and evangelical. But I'm not sure that is the case either. So how do I know if someone is evangelical? If I ask a Christian, will they know if they are evangelical? It seems absurd that we have this word that is thrown around so much but almost no one can tell me who exactly fits in that category.