Microstakes is a terminology, which is only used for
online poker. Most
poker sites define it as cash games with up to 15$ buyin, although somewhat confusingly at
pokerstars, when you set a filter, the 16NL tables show up as "micro". But on most other sites the highest cash games, which are considered "micro", are the 10NL tables. For live cash games the term "low stakes" would typically mean a 1/2 or 1/3 game (in dollars), since these are usually the lowest stakes offered by
casinos.
I also agree with others, that these terms are not really important. If for instance someone is sharing a hand history, the information presented should be "I was playing a 2NL full ring cash game" or "I was playing a 3,3$ PKO MTT" rather than "I was playing a microstakes game". Some people share
hands from 0,25$ MTTs, and I would not expect those to have the same kind of player pool as 4,4$ MTTs, even though both are considered "micro". Same goes for 2NL vs. 10NL cash.