Backtrack includes all of the known (and not so known) security tools used by hackers, pentesters and even script kiddies like: The Metasploit Framework, Burpsuite, Maltego, Dradis, Nmap, ng Wireless Cracking suite etc. Besides that, Backtrack also has different services integrated like httpd, mysql, ssh, tftpd, vnc etc all accessible from the menu on the panel.They are used for quick testing when auditing source code and other stuff. Backtrack also has wine already installed, to access windows drives and run windows programs.
Most of the time you will be accessing these tools through the menu on the panel. Selecting your tool of choice will either launch the gui of it (if available) or a terminal window will open with the working directory being the directory which actually holds the tool. Also when there is no gui, Backtrack will launch the tool's help page alongside the terminal window so you can instantly see the specific options to that tool.
Backtrack being now based on Ubuntu uses apt-get to keep itself up to date.It maintains its own repositories on offensive-security.com which are enabled by default. You can also add Ubuntu repositories but they need a little tweaking sometimes to make them run.