We pay US$10.00 (that's roughly €9, CAD$12, or AU$13) per review. Payments are made using PayPal (it's easy to sign up for a free account later if you don't already have one). A review is a few paragraphs (at least 375 words) about the hostel and some photos taken with your digital camera. You can see an example of a review on Hostelz.com here (look for "The Hostelz.com Review").
There is a minimum of two reviews per reviewer, but beyond that you can write as many as you like. You don't have to be a great writer, but you must have good basic English grammar and writing skills. Reviews are usually anonymous, but if you want credit for your reviews then let us know and we can include your name.