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.
1My take is that "quite" is more often used by speakers of BrE and "pretty" by speakers of AmE. Both words are ambiguous; ie, they don't represent specific grades that everyone would agree on. Even terms like "a couple of people" and "a few people" might mean the same small number (2-4 perhaps), depending on the speaker. More important for "pretty" and "quite" might be the stress used by the speaker. I don't think I'd ever stress "pretty" in "I'm pretty well", and if I stressed "quite" in "quite well", I might be expressing anger or a "none of your business" attitude. – None – 2013-02-06T16:21:21.977
2Disagree on pretty being not used in British English. It's pretty common to use it over here :) – Matt – 2013-02-06T18:43:01.680
This question makes me think of the fictional establishment "Ralph's Pretty Good Grocery Store" in the American radio show "Prairie Home Companion". It's certainly not an upscale establishment, but in its place, it's adequate. Quite usually implies something more upscale. – barbara beeton – 2013-02-06T20:31:13.520
@Bill Franke: Your US/UK division is absolutely correct - as that chart shows, pretty and quite are pretty much equally common in the Google Books UK corpus, whereas Americans favour pretty by a factor of over 4:1. But of course, affordances vary, and I couldn't have replaced pretty much with quite much there. Also don't forget the "typical British understatement" factor, whereby quite good often means not very good at all, actually.
– FumbleFingers Reinstate Monica – 2013-02-06T23:23:27.173