is the virtual memory space: the sum of everything in the virtual memory map (see below). It is largely meaningless, except when it isn't (see below).

is the resident set size: the number of pages that are currently resident in RAM. In almost all cases, this is the only number that you should use when saying “too big.” But it's still not a very good number, especially when talking about Java.

is the amount of resident memory that is shared with other processes. For a Java process, this is typically limited to shared libraries and memory-mapped JARfiles.