To enable you to search for packages both quickly and thoroughly, APT uses a local cache of the available packages. Try running this command:
matthew@seymour:~$ sudo apt-get update
apt-get update command instructs APT to contact all the servers it is configured to use and download the latest list of file updates. If your lists are outdated, it takes a minute or two for APT to download the updates. Otherwise, this command executes it in a couple of seconds.
After the latest package information has been downloaded, you are returned to the command line. You can now ask APT to automatically download any software that has been updated, using this command:
matthew@seymour:~$ sudo apt-get upgrade
If you have a lot of software installed ...