apt-cache is a part of the apt suite of tools used to manage packages in the Debian distribution. apt-cache is a tool for sifting through package information that resides on your computer after doing
Search for a package using a keyword:
apt-cache search mp3
This will search for packages that have "mp3" in the name or description.
Show package details:
apt-cache show mpg123
This shows package information about a package named "mpg123". It will display things such as the package dependancies, the long description, the version, the installed size, etc.