It's intellectually lazy to divide the world into two types of people, but when it comes to using computers to create music, it seems to be true.
The first group are what I'd call digital music enthusiasts. They compose almost exclusively at a computer, using a MIDI controller and/or sounds from a wide variety of third-party digital sources--loops from a program like ACID, beats and virtual synthesizers from a program like Reason, short samples that they recorded themselves or spliced from another source.
I'm a member of the second group, the reluctant analog dinosaurs. We came up playing instruments other than the piano or keyboard--guitar, bass, horns, drums--and value the spontaneity and collaboration of a live setting. To us, a night composing in front of a computer seems unbearably tedious. Instead, we use computers mainly in the recording process--either recording to hard disk, or mixing from another medium (ADAT or perhaps analog tape) to a program like ProTools. Of course, many people start in this group and eventually become full-fledged digital enthusiasts, but a lot of us approach computer-generated music with suspicion and even a bit of (often undeserved) scorn.
I was under the mistaken impression that Garage Band, part of the iLife suite that comes with every new Macintosh computer, is a tool for budding digital enthusiasts. A lot of the program's appeal is that you don't need to be a musician to use it--Garage Band includes dozens of built-in instrument sounds, with thousands more available through Apple's Jam Packs, and you don't even need a MIDI keyboard to enter notes, but can type them directly onto the typewriter keypad (the middle row of letters serves as the white keys, and the top row as black keys). It's trivially easy to import audio files or sounds from other programs such as iDrum, then cut and splice and loop and add effects.
But I recently realized how easy it is to use Garage Band for simple home recordings. Don't worry about an audio interface--sure, you'll eventually need one when you want to make higher-quality recordings or do live multitracking--but you can get started with a simple USB microphone. You'll have to be careful with the levels, and you can't record loud amplifiers or drums or it'll distort in ugly ways. (Use Garage Band's built-in drum beats or a program like iDrum.) You won't get a finished release-quality recording, but if you just want to hear how a few parts sound together before bringing them to the rest of the band, or need to make a quick sketch of a musical idea you got in the middle of the night, this is a cheap and very easy way to get started.