They're less labels for purpose and more labels for lifecycle - data you'd consider "cached" can appear just about anywhere. From what I remember (corrections more than welcome):
Active is fully-fledged in use memory, mapped into one or more processes
Inactive is where Active goes when it's less in-use. Cheap to reactivate, but relatively costly to free: can still be mapped into processes, and may be dirty (modified) and thus require writing to disk before being unmapped and cleared for reuse.
Cached is where lesser used Inactive cached data goes before it dies. More costly to reactivate, but cheap to free: No longer mapped directly into any process, and strictly consists of only clean (unmodified) pages that don't need writing back to disk before clearing.
Wired is pinned-down memory that can't be swapped out. ZFS's data and metadata caches are counted here, since it maintains its own (known as the ARC - Adaptive Replacement Cache, after the algorithm it's based on) instead of just relying on the traditional VM page cache.
