Memory cards are computer chips used to add memory to a computer.  Memory comes in two basic forms, random access memory (RAM) and read-only memory (ROM).  The name "random access memory" is a bit misleading, because both kinds of memory are random access; that is, the CPU can call up any arbitrary address directly without reading sequentially through lower addresses. RAM is really read-write memory, which the processor can use as a scratch pad and modify rapidly. ROM is memory that contains preset instructions and data, often for controlling physical devices like disk drives associated with the PC.  Configuration, performance specifications, and applications and features are all important to consider when selecting memory cards.