There are three different, but API compatible, implementations of a SLAB allocator in the Linux kernel. A particular implementation is choosen at configuration time.
SLAB: original, well proven allocator in Linux 2.6.
SLOB: much simpler. More space efficient but doesn't scale well. Saves a few hundreds of KB in small systems (depends on CONFIG_EMBEDDED)
SLUB: the new default allocator since 2.6.23, simpler than SLAB, scaling much better (in particular for huge systems) and creating less fragmentation.