There are lots of software packages inside the Ubuntu repositories; they are divided into categories as follows:
- Main: supported, free
- Restricted: supported, non-free
- Universe: unsupported, free
- Multiverse: unsupported, non-free
You may also add 3rd-party repositories.
To install/remove software packages:
- Click System -> Administration -> Ubuntu Software Centre (enter your password as needed)
- Mark the things you want to install/remove/upgrade/...
- Click apply
If you are using Ubuntu 11.04 or newer, your Ubuntu Software Centre is found in the applications directory.
To install 3rd-party software packages:
- Double-click on the downloaded .deb files
- Click install