roslib is the base dependency of all ROS Client Libraries and tools. It contains common tools like the generators for Messages and Services as well as common message definitions like Header and Log. It also contains the common path-bootstrapping code for ROS Python nodes and tools.
roslib for C++ Developers
roslib contains the definition of the ros::Time and ros::Duration objects used in roscpp and other ROS C++ libraries. It also contains functions for querying the ROS package system. Please see the Code API and the roscpp Time overview for more details.
roslib for Python Developers
If you are a Python developer, you might be confused that there are both the roslib and rospy packages that support Python development in ROS. In fact, some of the functionality they provide is very similar.
There is only one line of roslib you need to know, which exists for bootstrapping reasons:
import roslib; roslib.load_manifest('YOUR_PACKAGE_NAME')
Otherwise, the simple answer is use rospy. Unless you're more familiar with ROS Python development, rospy should provide you with all the APIs you need to run a node, call services, access the Parameter Server, etc... roslib is primarily an internal library used by ROS core developers to write tools.
Developer tools for client library authors
Currently no new features are planned for roslib. But as new, common functionality is added to the ROS platform, it will likely be supported by libraries here.