By definition, mnemonic devices are memory aids. They may be something verbal such as a very short poem or a special word used to help a person remember something. They could be visual, kinesthetic or auditory. They use associations between easy-to-remember constructs and the thing to be remembered. This is based on the principle that the human mind much more easily remembers something spatial, personal, surprising, or humorous as opposed to meaningless sequences.
To the right you will find a number of mnemonic devices people use to remember things.