SharePoint normally provides two types of logging types, the USL log and the Windows event logs. The ULS log is what you get in the C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS folder, and is often the first place you search for debugging problems with your solutions. These logs normally contain the bulk of the LIVE logging information provided by SharePoint that can be further filtered and sorted.

ULS Log Viewer is a free tool that can be downloaded from: