Process management and memory use are complicated subjects which are slightly different from one flavour of UNIX and UNIX-like systems to another. If you want to know more about them on your particular flavour (Linux, BSD, Solaris and so on), then these are possible starting points. This guide is heavily biased towards the Linux platform. The author would welcome corrections for this!
The manual entries for top, ps, nice, renice, df, du, id, getuid and free.
The info entries for df, du, and sync, all from the fileutils node, and for id in the sh-utils node.
The contents of /usr/doc/procps, /usr/doc/libgtop and /usr/doc/gtop (on Linux; these may vary in location on other systems.)
The document Memory Usage, by Miguel de Icaza and Jacob Bergman.