Mixing is taking a multi-track recording and balancing them using volume, eq and effects such as reverbs, delay and compression. The end result of this process is typically a 2 track format (stereo file that can be played on your mp3 player, cd player or dino-phone).
Mastering is taking a finished mix and using processors (generally eq's and compressors but sometimes others) to make the mix sound louder, fuller and cleaner.
The mastering engineer is the last person who works on your project before it's released to the public and therefore it's important you get the right person for the job. Now on a lot of studio blog postings this is where I would be shamelessly saying that person is me haha, but no, it's not.
Generally speaking the person you want to do the job is someone who has (for some reason) dedicated their life to mastering. It's a very specific art and not everyone can do it. Getting your producer or someone who "masters in-between recording bands" is the equivalent of getting a general contractor to do your plumbing. Sure they could do it, but a plumber could do it better, right? Right.
A lot of producers or engineers offer mastering to their clients but this should be avoided for three reasons:
1. As discussed above your producer/engineer isn't a dedicated mastering engineer.
2. The person recording/mixing your project should not be the one mastering. You need someone with fresh ears at this point to be objective.
3. Many times now albums have multiple producers. It's the mastering engineer's job to make the whole album sound like a cohesive piece of art (or commerce depending on who you are).
Now that I've given a basic primer on the difference between mixing and mastering and a bit of info on what to look for in a mastering engineer here's some links to two great mastering engineers I use for my projects on a regular basis. They are:
In addition to my own productions I work on for clients I do take on mixing projects as well. My website as always is here:
http://www.morphproductions.com Toronto Recording Studio