On vector machines, vector registers typically hold 64 elements, so long vectors must be stripped. A word is 64 bits, or 8 bytes. If there is only one load or store in a pipe, we say there is a 1-word memory bottleneck.
Memory bandwidth is often the crucial factor in determining the performance of a machine.
Main memory determines the size of a problem that can be solved on a given machine. Memory is measured in words or Mb. Extended memory is extra on-board storage, usually not as fast as main memory, but faster than disk access. With current technology, extended memory can be as fast as main memory, and is usually a couple of times bigger. Time to access cache is usually 1 or 2 cycles.