Acid 7 Pro is one of the most powerful pro-level DAW (digital audio workstation) packages on the market. Neither the feature set nor the price are for amateurs--but if you're serious about making and mixing music, this is one of the DAW programs to consider.
Coming from a loop-based (using repeating sample files to create songs) background, it takes a slightly different approach from heavyweights Steinberg's Cubase and Cakewalk's Sonar which started life as MIDI sequencers. If you're working with and recording audio snippets, you might prefer Acid 7 Pro. If you use MIDI and record continuous performances, you might go for one of the latter programs.
Acid 7 Pro's feature list is far too extensive to list here, but the major capabilities include tons of loop-based music creation editing tools, multi-track audio recording and midi sequencing (including support for VSTi virtual instruments), video tracking, multiple bus output and inline track editing. If you don't know what all those are, you might check out some less expensive music software first.
Personally, I don't like to register my name more than I have to (and use a false name generally when I do) so I didn't appreciate the trial version's insistence on performing said procedure before using the program. However, the fully functional demo shows off an easy-to-use interface and the program's multitude of pro-level features. Other loop-oriented programs in this category to check out are Ableton Live and FL Studio (formerly Fruity Loops. No, I'm not kidding).