P4V, the Helix Visual Client, is the cross-platform graphical user interface for the Helix Core server. To use the Helix Core server to manage files, you typically connect to it using an application like P4V. P4V enables you to check files in and out, and perform various other versioning tasks.
See the P4Admin User Guide for information on how to use P4Admin, a GUI for administrating Helix Core connections, depots, users, and groups.
See the P4Merge User Guide for information on how to use the visual tool for diffing and merging files.
See the P4V Cheat Sheet for a quick reference of P4 command and P4V icons.