It's funny that English is not technically a tonal language when so much of our speech depends largely on tone.
Quite means what you think it does. It basically means very. For all intents and purposes as a learner, you can think of it as a synonym for very or really.
Pretty, however, depends on the tone of the speaker. In general it's like a medium version of very, however, it can also mean not very.
I know that sounds completely nonsensical, that it can mean two things that are total opposites, but you will almost always know the difference when you hear it. Example:
Hey Nick! How are you?
Pretty good....I guess...
Is the job finished?
Pretty finished, yeah.
When said this way, you'll hear some doubt or hesitation in the speaker's voice, as if they're lying to you and not trying to hide it, and you will understand that what they're saying on the surface is alluding to something else underneath.
This usage is exclusive to spoken language and dialogue.