Top stands for Table of Processes. It is a program that shows you a list of processes running on the system, along with system statistics on load average and available memory. The display refreshes about once a second.
top is useful for seeing what processes are bogging down your system performance, or finding out the overall health of your system's processor and memory.
Pressing 'M' (capital-M) tells top to sort by memory usage. Besides the obvious reason, this is useful to keep the list in approximately the same order from update to update, making it easier to keep track of a given process.
To view a summary about the user/system/io load of each logical processor, type 1. You can get back to the normal view by typing 1 again.
r or n... then pid
press k, then type the pid
To quit top, type q.