What Is Log4Net assembly

Hi

As soon as i drag and drop crystal report viewer, the log4net assembly is being added to web.config along with some crystal report related assemblies.

This is the exact line in web.config file:

<add assembly="log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=692FBEA5521E1304"/>

Please help me know, what is this assembly and what it does. I've not found any relevant info on web about this.

Thank You.