Audacity is a free audio editor that was developed by The Audacity Team.
It can import/export many file extensions (WAV, AIFF, MP3, Ogg Vorbis) and with extensions of the program you can even import/export (FLAC, AAC, AMR, AC3).
You can easily cut, copy, paste, delete and crop audio files quite easily with your easy-to-use tools.
You can even make use of the VST Enabler, which enables you to use VST/VSTi.
You can split audio files and apply basic effects to the files such as pitch/tone/amplitude/noise-maker.
If you add in extensions/plug-ins to the software, you can also make use of Nyquist.
Audacity is a great free program to get you started on audio-editing/producing. You have to keep in mind, however, that the program is rarely intended for professional use. It is a basic program that serves many needs but will not suffice when one would like to record and produce audio files professionally.