Row counters are the general name for a variety of devices that are used to help keep track of which row you are on in a knitting pattern.
There are three basic varieties, all pictured above. At left is a barrel row counter that can be slipped onto your knitting needle. The counter numbers turn individually, making it easy to count backward or forward, and keeping the counter on the needle means you're less likely to lose it.
The second variety, pictured in the center, is commonly known as a kacha kacha. This device has a button on top that you press to move the counter forward (the sound it makes is something like kacha kacha). You can also move the numbers individually by hand if you need to.
These devices tend to be bigger than needle barrel row counters, though they often have a place to thread them onto a necklace so you can wear them around your neck. They also have the benefit of including a lock, so it's difficult for someone to mess up your row count by playing with the device.
Finally, at right is an electronic row counter. This particular model, from Nancy's Knit Knacks is mounted on a ribbon to wear around the neck. It has an on-off/reset switch as well as up and down buttons. To keep your count you have to keep the counter on for your entire project, but the manufacturer says this does not produce an excess drain on battery life.
If you want to go completely low-tech you can use a piece of paper as a row counter, just jotting down which row you just finished when you stop knitting or marking a hash mark for each completed row.