At Mann's Music we sell computer music software for the musician that wants to get started doing some home recording or even notating and composing their own music.
Steinberg's range of Cubase products are excellent and ideal for recording your music onto a computer. There are many benefits of this such as easy editing of parts, no costly studio time and being able to work from the comfort of your own home studio. You can record an idea, layer it up with different parts, mix it down and then upload it to the Internet or burn it to a CD. You can even add drum machines and synth parts to your songs using VST instruments, many of which are available as free legal downloads.
The Sibelius software focuses less on home recording and more on notating and scoring ideas. This is great for musicians that want to document their compositions. It is also excellent for those who want to write and compose whilst on the computer. You can simply notate a part in Sibelius and then allow the computer to play it back using realistic sound samples, giving you a good idea of how your music would sound with a real orchestra and instruments playing it.