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Sound Editors - generally used for the manipulation of single mono or stereo sound files, application of sound effects, and exporting in various formats and qualities.

Studios, Sequencers and Composers - multitrack programs which allow you to combine several different sound files or audio sequences to create songs.

Software Synthesizers - keyboard/synthesizers for the computer. Virtual sound generators, usually including a bunch of sound manipulating tools and effects.

Sound Converters - Programs that allow the conversion of sound files from one format (eg wav) to another (eg mp3). Most Sound Editors are capable of outputting various formats, so if you have a decent sound editor you're probably set.

Special Thanks to Denvish for organizing the above information.

Also see Denvish's list of Audio Tutorials!