Audacity is an Open Source audio application for Windows and Linux designed to be a full-featured yet easy-to-use multi-track recording and editing suite.
Audacity can record live audio through a microphone, mixer, digitize recordings from other media, and capture streaming audio. Audacity can record, import and export, modify quality, has a ton of built-in effects, analyze capabilities, plug-ins, and just so much more, we're scratching the surface.
It supports almost all of the popular audio formats out there, including WAV, AIFF, Ogg Vorbis, FLAC, MP3, and FFmpeg-compatible files. Oddly enough, the one format it does not support is M4a, which is popular with Apple and iTunes. If your music library has many songs you downloaded from iTunes, you're going to need to download a converter first.
Audacity offers some support within the application. It includes a quick help (local or the internet), a user manual online, a Wiki page with tips, tricks, and more, and a link to their online forum.
We're not going to claim to be audiophiles or sound engineers, far from it, but we feel Audacity is every bit as good as some professional applications on the market.