What is the difference in meaning between these two words and how does their usage differ?
Чересчур is one of those words that don't sound bookish, and yet you're radically less likely to hear it in everyday speech compared to слишком. Чересчур just isn't the word you'd immediately think of when seeking to express the idea of excess. But when you do hear it, it sounds perfectly natural and plain, "oh yeah, that's a word too."
Also I feel subjectively that слишком is more, as it were, focused. All other things being equal (i.e. discounting the abovementioned general likelihood to think of слишком as a word choice before you think of чересчур), you're more likely to pick слишком over чересчур if you think you can more or less point to the exact measure, or the exact cause, of the perceived excess. Чересчур is more of a "dunno, there's just too much of this here thing."
That said, это уже слишком "now that's enough!" makes you sound annoyed or angry while это уже чересчур is a more laid-back "now that would be going too far".
Well, the difference is in negative form:
For example, Он не слишком умён means He's silly. Yet negative не чересчур is almost never used except questions: А не чересчур ли он умён? etc.
Also с лишком literally means with excess, so if written as a whole слишком is an adverb, but separate с лишком is a noun.
The etimology of "чересчур" is much more vague. First of all, of course, чересчур = через чур, but what is чур, anyway?
Certainly, we have the word чур as a part of "a guard spell" against the evil: чур меня! Also consider the verb чураться (to keep away, to shun). Still it helps a little, because the meaning of it is unknown. One option could be, for example, Чур / Щур (cf. Пращур) = Ancestor (so Чур меня! means smth. like Ancestors [defend] me!), but Vasmer dictionary gives this as "unlikely".
Another point is that чур меня! and чересчур may be not related in any way. Vasmer says that the word Чур, meaning a kind of Limit or Bar, is to be reconstructed from the word Чересчур. Personally I know only one appearance of the word чур in this sense: Чуры не знать means To have no limits (especially when someone eats too much). But it's a dialectal expression, and I'm not sure if чересчур is really related to чуры не знать.
If I tried to compare translations, I'd say that 'слишком' is 'too' and 'чересчур' is like 'too much' or 'more than enough'. For instance: 'Он слишком умный' = 'He is too smart' and 'Он чересчур умный' is 'He is smart more than enough'.
Btw, with 'smart' expressions above sound a bit depreciatingly.
Слишком is too serious while чересчур is only slighly disapproving as if you were approving it in essense or in the deep of your mind or it is generally a good property, just here it is unusually excessive. Like in the case of playing children:
Вы слишком шумите! - You are producing too much noise, reduce it!
Вы чересчур шумите! - Your noise is excessive. Even though I like it and think you essentially have the right to do it, it can be annoying for other people around.
У вас слишком большой рост. - Your height is too large. You do not fit in the requirements. It is unlikely you will pass.
У вас чересчур большой рост. - Large height is good in general, and I like(we look for) specifically tall people but I am a little concerned about yours because it may be excessive. Nevertheless, we can give you a try.