P4V, the Helix Visual Client, is the cross-platform graphical user interface for the Helix Core Server. P4V enables you to check files in and out, compare versions, work with streams, and much more.
See P4Admin User Guide for how to use P4Admin, a GUI for administrating Helix Core connections, depots, users, and groups.
See P4Merge User Guide for how to use the visual tool for diffing and merging files.
See P4V Cheat Sheet for a quick reference of P4 command and P4V icons.