Extensions
Software that increases your Mac's capabilities is called an extension.
Extensions show up at the bottom of your screen as your computer boots up.
If an extension loads incorrectly or is turned off, a black or red "X" will appear over it as it shows up on the screen.
When installing printer software, an extension may be loaded automatically.
Some common extensions:
Apple CD-ROM
Apple Guide
Apple Photo Access
AppleShare
Find File Extension
Finder Help
Print Monitor
Printer Share
QuickTime
Sound Manager