What they said...

But I would also add that the term "net" can ALSO include costs associated with producing the book. So, the cover price, less any discounts, LESS the cost of paper and ink and cover art and salaries, etc., etc. Seek out contracts that offer royalties on "list" price, "cover" price or "gross."
And while we're talking about royalties and advances, let's talk a bit about some other stuff that often confuses people. First, more and more publishers are starting to stagger advances, doling them out in sections until the book comes out. It used to be that an advance was paid when the contract was signed. But then some authors didn't turn in the work and the publisher realized that they'd (horror of horrors...

) actually
spent the money to live on. So, a lot of the big houses are beginning to offer the payment in half or thirds. You get a chunk when you sign, and another chunk when they accept the manuscript (which is NOT, by the way, when you first send the manuscript to the publisher. It's AFTER edits are completed satisfactorily), and another chunk when the book hits the shelf.
Now, once the book hits the shelf, you have some waiting to do. Most publishers work on a twice-yearly reporting schedule, meaning that they will tally sales and returns for credit over a six month period and report the ACTUAL sales. On a January-June reporting period, you'll probably see the first statement about the sales come September or October. For July-December, it'll be March or April of the next year. IF a book "earns-out" or "sells-through," you'll get a check in the FOLLOWING reporting period, because most publishers will hold out a period of time for bookstores to report back on unsold copies.
So, let's say you published a book in January. Come October of that year, you'll get the first statement about how many copies were sold from January-June. But you won't see any money, even if they all sold and you earned out. The publisher will hold a "reserve against returns" until the NEXT pay period. So, if no other books sold from July-December, and no extra ones were returned from the sales on the first period, then come April of next year, you'll see the royalty money.
In addition, if you published in May, often there's a contract clause that says they don't have to report AT ALL during the first royalty period (because it's too much bother to accumulate sales for only one month -- May-June). So you won't see even a statement until April of the next year.
It's frustrating for an author, because we have no way of knowing how the book is doing until we see it on paper months and months from the date of release. Sigh...
But, that's how it works, FWIW!