A log, in the computer world, is a document of events in a specific program. It may list errors, warnings, status, and execution details.
In the case of "HiJackThis" (HJT) logs, which are what Malware and Spyware Removal uses quite often.
The HJT log details system information, such as startup entries, browser helper objects (BHO's), and various other bits of information that can help identify and remove a virus, spyware, or adware program.