The general definition  is a solid state (i.e. No moving parts) memory chip, or series of chips linked together, that holds the data stored on it, without the need of an electrical supply to keep that Data stored, like RAM (Random Access Memory). 

The most common use for it is a USB pen Drive, of which there are to many to mention here.Most range from about 1GB upwards. The other uses are for Digital cameras, and now movie cameras.

Types such as: SD, XD, SMART, MEMORY STICK, etc. They are being used to replace the traditional, (moving parts) Hard Drive in the computer, thus making them more reliable.