top - 16:05:14 up 13 days, 6:35, 4 users, load average: 0.07, 0.04, 0.02
Tasks: 107 total, 1 running, 106 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.7% us, 0.1% sy, 0.1% ni, 98.8% id, 0.2% wa, 0.0% hi, 0.0% si
Mem: 255608k total, 243632k used, 11976k free, 10276k buffers
Swap: 522072k total, 59284k used, 462788k free, 102412k cached
PID USER PR NI VIRT RES SHR S \%CPU \%MEM TIME+ COMMAND
1 root 16 0 1740 500 472 S 0.0 0.2 0:00.67 init
2 root 34 19 0 0 0 S 0.0 0.0 0:00.01 ksoftirqd/0
3 root RT 0 0 0 0 S 0.0 0.0 0:00.00 watchdog/0
4 root 10 -5 0 0 0 S 0.0 0.0 0:03.05 events/0
5 root 19 -5 0 0 0 S 0.0 0.0 0:00.01 khelper
6 root 11 -5 0 0 0 S 0.0 0.0 0:00.00 kthread
10 root 10 -5 0 0 0 S 0.0 0.0 0:00.46 kblockd/0
11 root 20 -5 0 0 0 S 0.0 0.0 0:00.00 kacpid
252 root 20 -5 0 0 0 S 0.0 0.0 0:00.00 khubd
256 root 10 -5 0 0 0 S 0.0 0.0 0:00.00 kseriod
313 root 15 0 0 0 0 S 0.0 0.0 0:00.18 pdflush
314 root 15 0 0 0 0 S 0.0 0.0 0:00.11 pdflush
The first few lines tell you useful information about the machine. These include: the current time, how long the computer has been running for, how many users are using the machine, what the load average has been (more later), how many tasks are active at the moment (and how many are actually running), how much of the CPU's time is being used, how much RAM the computer has, how much is being used, and how much is still available.
After the first few lines texttttop has a list of processes and various pieces of information about them. Probably the most important things for now are the PID (first column), who's running the process (second column), what percentage of the machine's CPU time and memory it's using (9th and 10th columns), how long it's been running for (11th column) and what the process is called (last column).
Pressing `M' causes top to display the top processes in terms of memory usage, rather than CPU time. You can return to CPU time by pressing `T', and `q' quits.