What is the "Subjunctive"? And when should it be used?

The subjunctive is what's known as a "mood", which merely means that it's used in a different way to "normal" (indicative) verbs. That is, in most cases, to express an element of doubt. However, it need not be translated into english in any special way.

In Latin, there are a couple of times the subjuncitve is used: as a hortatory (Let's go!), and in certain gramatical constructions like result clauses.