Unfortunatly "stackable" is one of those terms that different vendors have used for different things. It originally came from hubs where 2 or more stackable hubs joined together would act as one logical hub (avoiding the 5-4-3 rule limitations). The hubs would usually have a special "stack" port on the back with a multi-pin cable joining the hubs together. When switches came along, different vendors tried to apply the term "stackable" to the switches in different ways. It usually means that you connect the switches together through a special cable / port and the whole cluster of switches is managable through one IP and interface (so four 48 port switches act like one big 192 port switch). Other vendors just mean that there is a special (high-speed (2.5-8Gbs)) interface that hooks the switches together (often bus-to-bus). One (infamous) vendor used the term just to mean that you could physically place one switch on top of the other and the feet on the upper switch would fit into indentations on the lower switches!
Cascading switches just means hooking one switch up to another through normal (usually Ethernet) ports. You still get the ability to have more ports, but it is often slower and you lose the single interface / IP managability (which in my opinion if of limited use).
