I don't have any info on interpreting error logs (as I haven't looked at mine yet), but I can decipher the example you cited.
Basically, that's an HTTP request header sent by a web agent (browser, robot, etc.). The "GET" part is the agent stating it wants to receive a particular resource. In this case that resource is "/cgi-bin/cframes/compose". All the stuff after the "?" are arguments to be passed to "compose". If you split that junk on the "&" you are left with:
The HTTP/1.0 part is the agent telling the server which version of HTTP it would like to use.
Anyway, "compose" should have variables named disk, login, f, curmbox and _lang. Presumably, compose can take the values assigned to these, do some magic and return something to the agent.
This probably showed up in your error log because there is no "/cgi-bin/cframes/compose" resource at your site.
This may be more info than you need, but I hope it helps. I've just learned about HTTP myself, so there may be other people who could shed more light.
For more info on HTTP, head to www.w3.org
or pick up "Web Client Programming with Perl" by O'Reilly.
[This message has been edited by fuddmain (edited 06-16-99)]