MAC TOP manpage

            pid    Process ID (default).

              command
                     Command name.

              cpu    CPU usage.

              csw    Number of context switches.

              time   Execution time.

              threads
                     alias: th
                     Number of threads (total/running).

              ports  alias: prt
                     Number of Mach ports.

              mregion
                     alias: mreg, reg
                     Number of memory regions.

              rprvt  Resident private address space size.

              rshrd  Resident shared address space size.

              rsize  Resident memory size.

              vsize  Total memory size.

              vprvt  Private address space size.

              kprvt  Private kernel memory size.

              kshrd  Shared kernel memory size.

              pgrp   Process group id.

              ppid   Parent process id.

              state  alias: pstate
                     Process state.

              uid    User ID.

              wq     alias: #wq, workqueue
                     The workqueue total/running.

              faults alias: fault
                     The number of page faults.

              cow    alias: cow_faults
                     The copy-on-write faults.

              user   alias: username Username.

              msgsent
                     Total number of mach messages sent.

              msgrecv
                     Total number of mach messages received.

              sysbsd Total BSD syscalls.

              sysmach
                     Total Mach syscalls.

              pageins
                     Total pageins.

Definitions
Now that we know roughly how the stuff works, let's define some memory-related terms:


Resident: memory which is located in physical RAM.
Private: memory which is only mapped into one process.
Shared: memory which is mapped into multiple processes.
Address space size: the quantity of address space occupied by a particular section of virtual memory.
Memory size: the amount of actual physical memory occupied.
And with that, we can now see what the various fields in top mean, from looking at the man page and using these definitions:
RPRVT: The amount of address space, local to this process, which corresponds to items currently present in physical RAM.
RSHRD: The amount of address space, shared between this process and at least one other, which corresponds to items currently present in physical RAM.
RSIZE: The total amount of physical RAM used by this process. (This is not equal to RPRVT + RSHRD because they measure address space, but this measures actual memory.)
VPRVT: The amount of address space in the process mapped to items which are not shared with other processes.
VSIZE: The total amount of address space in the process that's mapped to anything.