The division is mainly made according to boat shape and purpose.
Regarding boat shape you can simplify:
long + slim = wobbly + fast
short + wide = stable + slow
Further distinctions can be made regarding the equipment of the kayak. If you are looking for a kayak for tours with luggage, then all-round touring boats or fast single kayaks are better suited, as they have certain standards of equipment, like water-tight storage and perfectly adjustable seating position.
But a basic rule is: Test first, then buy!