Single units, such as up, off, over, down, under, for etc are prepositions.
Combined with the verbs, they add different meanings to them.
Such verbs become phrasal verbs: turn off/on, put out, get over, break up with, come up with, go through etc
e.g. You may switch the light off now. I must turn my computer on in order to use it. He came up with a brilliant idea. I broke up with my boyfriend last week.