From PubMed:

At least two types of memory cells have been defined in humans based on their functional and migratory properties. T central-memory (T(CM)) cells are found predominantly in lymphoid organs and can not be immediately activated, whereas T effector-memory (T(EM)) cells are found predominantly in peripheral tissue and sites of inflammation and exhibit rapid effector function.

Actully though, it remains unclear whether these represent two distinct subsets of memory cells or two different phases, since memory cells may acquire and shed cell surface markers as needed.