GarageBand very much fits the mould of a very simplified and intuitive approach to something that is traditionally quite complex - it's to music editing what iMovie is to video editing. The interface for GarageBand is very straight forward, and most functions are ready to hand. Setup is also very simple, with little need to worry about things like MIDI in/out/through or ASIO drivers and such.

At the core, there are three things you can do with GarageBand: record Real Instruments (audio tracks), Software Instruments (MIDI tracks), and add prerecorded loops - then mix them all together. You have over 1,000 loops to play with, some being MIDI-based and others audio, and there is a wide range of styles and instruments to choose from. While this allows you to immediately get stuck in and put together a whole song without having to actually record anything, you quickly find that many of the loops are incompatible, such as drum lines.

GarageBand does include over 50 MIDI instruments, some of which are impressive, as well as many customisable effects for audio tracks, such as chorus, flanger, distortion, amp stimulation, echo, reverb and more. These effects are very easy to manipulate, and are quite powerful, but once you've chosen your effects, they're fixed for that track from end to end. So while there's a surprising amount of power beneath the surface in GarageBand, you do end up being quite limited in terms of flexibility.

Editing tracks is very simple and intuitive, and even a total PC novice should have no trouble getting the hang of mixing a number of lines together in a few hours. You can also edit individual MIDI notes and have rudimentary control over the audio tracks. You output to AIFF format to iTunes, but then you can convert to MP3 or ACC from there.
At the end of the day, GarageBand excels at its goal of bringing music creation to the masses. Other music programs on PC and Mac tend to be very complex beasts, and very alienating to the average user. So, if you want to play around with music, or even test the water to see if you have what it takes to be a pro, GarageBand is the perfect choice. It's only available on Mac, though, so in lieu of Apple porting it (very unlikely...), PC users can only hope some PC developer will draw inspiration and deliver something of a similar intention and calibre.