Greg says what it’s not, and since he didn’t use the opportunity to also succinctly express what it is, I helped him out in the comments:
CQRS (“Command-Query Responsibility Segregation”) is a simple pattern that strictly segregates the responsibility of handling command input into an autonomous system from the responsibility of handling side-effect-free query/read access on the same system. Consequently, the decoupling allows for any number of homogeneous or heterogeneous query/read modules to be paired with a command processor and this principle presents a very suitable foundation for event sourcing, eventual-consistency state replication/fan-out and, thus, high-scale read access. In simple terms: You don’t service queries via the same module of a service that you process commands through. For REST heads: GET wires to a different thing from what PUT/POST/DELETE wire up to.
Martin Fowler has a nice discussion here, with pictures. Udi Dahan has another nice description, also with pictures. To say it in yet another way, the key point of the pattern is that the read and write paths in a system are entirely separate.