User, System, Nice, Idle, Wait, and I don't know what the last couple are. (I know these just from using them; not sure where that's actually documented!)
"sar" is probably a better tool for collecting that type of information; it will by default give you a historic list rather than a dynamic display which should help you more for troubleshooting. (sar comes with the "sysstat" package on CentOS; not sure about other distros).
"iostat" (part of the same package) is also better than "top" for showing you instant system stats for data collection. You can run for example "iostat -c 2 5" for showing you CPU utilization stats (including iowait), collected at 2-second intervals for 5 intervals.
"top" is really best as an interactive tool; it suffers from information overload if you're just trying to get some simple numbers. Of course all these statistics are collected by the kernel itself; these userland tools just retrieve those numbers from the kernel, so there is no difference in accuracy with any of them.