The CSlib client/server library is licensed under the modified Berkeley Software Distribution (BSD) License. This is a very non-restrictive license which basically means the library can be used by anyone for any purpose, including commercial ones.
The legal text for the license is in the LICENSE file included in the CSlib distribution. As stated there, the CSlib library comes with no warranty of any kind. The copyright notice is listed in the header of each source file.
See www.gnu.org or www.opensource.org for a discussion of the open source philosophy.
See this page for ways you can contribute to the CSlib.