NOTCE: CC-Get is a third-party program and is not built-in to ComputerCraft.
CC-Get is basically an installation, updating, and removing wizard for programs in ComputerCraft, similar to apt-get on linux (Advanced Packaging Tool).
Once a creator adds their work to the CC-Get website, you can use the program to install it using the install command. CC-Get requires the HTTP API, which needs to be enabled in ComputerCraft's configuration.
The website for CC-Get is here.