Linux Top Command Details

PID -- Process Id
The tasks unique process ID, which periodically wraps, though never restarting at zero.

USER -- User Name
The effective user name of the tasks owner.

PR -- Priority
The priority of the task.

NI -- Nice value
The nice value of the task. A negative nice value means higher priority, whereas a positive nice value means lower priority. Zero in this field simply means priority will not be adjusted in determining a tasks dispatch-ability.

VIRT -- Virtual Image (kb)
The total amount of virtual memory used by the task. It includes all code, data and shared libraries plus pages that have been swapped out.

RES -- Resident size (kb)
The non-swapped physical memory a task has used.

SHR -- Shared Mem size (kb)
The amount of shared memory used by a task. It simply reflects memory that could be potentially shared with other processes.

S -- Process Status
The status of the task which can be one of:
D = uninterruptible sleep
R = running
S = sleeping
T = traced or stopped
Z = zombie

%CPU -- CPU usage
The tasks share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time. In a true SMP environment, if Irix mode is Off, top will operate in ´Solaris mode where a tasks cpu usage will be divided by the total number of CPUs. You toggle Irix/Solaris modes with the I interactive command.

%MEM -- Memory usage (RES)
A tasks currently used share of available physical memory.

TIME+ -- CPU Time, hundredths
The same as TIME, but reflecting more granularity through hundredths of a second.

Command -- Command line or Program name
Display the command line used to start a task or the name of the associated program.

Note: Of course you can get more info on this from 'man top'