A small GTP binding protein involved in vesicle trafficking within cells.

So far over 60 different Rabs have been identified, including numerous subtypes e.g. Rab5 a, b and c.  However, there may be many more Rabs that have yet to be discovered.

Different Rabs mediate transport of different vesicles and cargo to specific targets through different pathways.  The large number of Rabs allows for specificity in vesicle transport.

Yes, there are many diseases which are thought to be caused, at least in part, by defects in Rab pathways.  These include:

Hermansky–Pudlak syndrome - Similar symptoms to Griscelli Syndrome; also thought to be caused by mutations in Rab 27.

Charcot-Marie-Tooth type 2 disease - An autosomal-dominant axonal disorder associated with severe sensory loss, thought to be caused by mutations in Rab 7.

X-linked non-specific mental retardation - Thought to be caused by mutations in the GDI 1 gene.

Warburg Micro syndrome - Related to defects in the eye.  Thought to be caused by defects in the Rab 3 pathway.

Without Rabs it would be very difficult to mediate specific vesicle trafficking.  Each Rab binds to a specific Rab Effector Protein (REP) on the acceptor membrane (the membrane "accepting" the vesicle). 

Because of this, Rabs can only bind to certain membranes e.g. Rab 7 can only bind to the membranes of late endosomes.  This means that any vesicle with Rab 7 bound to it can only dock at a late endosome, meaning its contents will eventually be degraded by the cell.