Audacity is an editing program which allows you to record and tinker with audio. Not only can Audacity record audio simultaneously from various inputs (for example, a USB microphone), it can also trim and edit clips. Furthermore, it supports multiple tracks, allowing you, for instance, to record lyrics and backing music separately.
The software also supports a number of audio effects such as noise reduction, as detailed in its extremely comprehensive manual which is both bundled with Audacity and available online. Audacity also supports VST (Virtual Studio Technology) plugins. Tracks can be exported in a number of popular sound formats such as WAV, OGG and MP3. If you want to know more about Audacity, then have a read of our full review here.