A phrasal verb is a verb with two parts; the verb and a preposition. The preposition changes the meaning of the verb - to turn on a light is different from to turn a corner.
Phrasal verbs are very difficult for English language learners because there are no universal rules. There are, however, a few guidelines that can help - see here: http://tiny.cc/fxkge
Ultimate Phrasal Verb Book
English Phrasal Verbs in Use: Advanced
Oxford Phrasal Verbs Dictionary (Elt)
Learn 100 Most Useful Phrasal Verbs in English in Six Easy Steps.