What is "Mixing"?

I’m often asked by clients what I mean when I say a song is ready for “mixing”. They usually assume after they’re done recording that it’s finished. And it can be! But usually there is a lot that can be done to turn it from good to great. There are 3 main phases in recording a song/CD. Tracking, mixing, and mastering.

Tracking is the first phase, where you’re literally recording each track of the song. Even though there might be some processing involved, such as compression and EQ (more on this later), it’s generally not considering “mixing”.
Here is an example of a song after the tracking phase:

Notice how the sound field is completely unbalanced. The drums and vocals are off in a distance, the bass almost non-existent, and the guitars very loud. Everything seems mushed together, nothing is very clear.

Mixing occurs when we edit and process each track after it is done being recorded. This phase usually takes the longest, but is the most beneficial for the song’s quality. The goal of mixing is to get each track to sound its best and fit in with the rest of the tracks. This is done through the use of EQ, compression, reverbs/delays, panning, automation, etc.
Here is an example of a song after it has been mixed:

Now notice how everything seems more balanced. You can hear every instrument, and nothing seems to be fighting for space as much. Some effects have been added for a more produced sound, and with the use of compression and automation, the music flows, rather than staying the same volume.

Mastering is the final stage. True mastering shouldn’t be done by the mix engineer for a variety of reasons I won’t go into now. Where before the song consisted of many individual tracks, it is now “mixed down” into one stereo track. Similar processing takes effect in this stage as in mixing, but affects the overall song, not just one track. Mastering generally consists of EQ, Compression, and Limiting, although other processing is often used.
Here is an example of a song that has been Mastered. Note- I did this in 5 minutes and on headphones. It is far from the ideal way to master anything, but I'm using it for demonstration:

Notice how the song is more glued together. It's also a bit louder than the mixed version so it can compete with songs at similar volumes. The bass is a little more prominent, the stereo width has been widened slightly, and some high end has been added to give it some air. Though I didn't do it for this master, it also requires some DeEssing, as you might be able to tell from the harsh sound whenever an S is sung.

Of course there are exceptions to every rule. Some songs don’t need mixing, and some don’t need mastering. But hopefully this has answered general questions about the stages of your project.