
The Contract. This is often pretty straightforward, and since the advent of the underground indie labels, it's even moreso: you'll probably get a straight split of the profits, usally 50/50 with the label. It's not a certain amount per unit sold (that's for the major labels and the old days) - but make sure the contract confirms that any expenses (mastering, artwork, PR) come out before you split the cash. Some labels try to take all this out of the artist's share. Naughty
Remixes are another major way of getting your name around, and for many people it's one of the first ways to get their work released. Let's assume that you are the remixer here - what are your rights? What do you get? Sadly, remixing is a bit of a thankless task these days. You won't see a contract unless you're remixing for a huge label, and in 99% of cases you're not really entitled to anything beyond what you initially agree. The label own all the rights to the original track, even if you've completely rewritten the whole thing, so they're not obliged to give you a cut of sales (and it would be incredibly unusual if they did). So beware of remix competitions - while many of these are very good and have great prizes, for some unscrupulous labels they can just be a cheap way to get PR and a free track to sell. Read the small print!

So far, we've just talked about sales money - but there's a whole other side to the equation. You'll often hear talk of 'royalties' but this can mean two things; money made when someone buys your track, and money from when your track gets played on the radio or TV (or even Spotify or Youtube!). This is 'publishing' royalties, and if your track does well it can add up to a tidy amount - in the UK at least, a single play on one of the BBC national radio stations can bring in more money than you'd get from scores of sales on iTunes.
How do you collect this money? There are two ways. The first is to have a publisher - but that's maybe something for a bit further down the line when you have a few releases under your belt. Your publisher will collect all this money and pass it on to you. In the meantime, you'd be well advised to register with your local collection agency. These are different for each country - in the UK we have the PRS, in the US it's ASCAP and BMI - and while there is sometimes a joining fee, it's usually something you'd make back with your first release. You register with the appropriate society, then when you have a track out, you register it on their database. They'll then look out for money owed to you.

So, before you dip a toe in the world of releasing music, it's worth getting a little familiar with some of these concepts, so at least you can go in armed with knowledge! Now, get sending out those demos...