Succinctly, a ripple carry adder passes its carry bit through a long logic chain, which is very straightforward to design, but can have a very large delay.
A carry-lookahead adder uses a clever algorithm to cut that logic to only a few layers, and thus keeps the delay pretty short even for a worst-case situation.
**broken link removed** of the carry lookahead algorithm is something that often shows up on places like Digital Logic exams, in case you're curious.