The basic difference between these two types of intelligence is that fluid intelligence has more to do with problem solving in unfamiliar situations while crystallized intelligence has more to do with the knowledge that one has acquired and one's ability to apply that knowledge.
These two types of intelligence are not completely separate from one another. A person who has more fluid intelligence is also more likely to be able to acquire more crystallized intelligence. The ability to think, that is, tends to help one to acquire more knowledge.
Overall, then, fluid intelligence is correlated more with problem solving while crystallized intelligence is more closely correlated with using one's knowledge and experience.