What can you do with GarageBand?
It's a great application to use for creating your own music or podcast.  It's like having your very own music studio right on your Mac.  Thanks to a range of built-in instruments and sound loops, you'll be able to create, record, and put together your own music masterpiece and easily share it with others.  You can also work with multiple audio tracks and audio filters.

Go Virtual
GarageBand's Magic GarageBand feature allows you to play several virtual instruments thus giving your composition it's own guitarist, bassist, drummer, keyboard player, and/or melody in addition to your own instrument.

Use
GarageBand features a multi-track drag-and-drop interface (timeline) allowing users to place together different loops, recordings, or tracks.  GarageBand uses 'projects' that hold your music and keeps tracks of changes you make to your composition.  Within a project you can use the included Apple Loops or your own recordings made via a microphone, real electronic music instrument(s), or built-in software instrument(s).  GarageBand makes it easy to arrange, mix, and add effects to your music.  Once your composition is complete, you can share your project with another iLife application (such as iTunes), burn it to CD, or add it to a video project.

You can also extend GarageBand via GarageBand Jam Packs or third-party additions.