Phrasal verbs are verbs of more than one word, usually two words, but sometimes even three. Everyone knows a few phrasal verbs: come in, go up, pick up, etc. The ones that most people know are the commonest ones. Native English speakers often use a much wider range of them. In general, however, they're used to define a more specific action than the single verb itself: come vs. come in, or go vs go away. Phrasal verbs are quite common in spoken English and less frequent in formal correspondance. You can also refer to phrasal verbs as multi-word verbs.
A TYPICAL PHRASAL VERB = [verb] + [particle] (the particle is either an adverb or a preposition, or both).