There are three basic ways to get water flow: internal pumps (aka powerheads), external pumps, and surge devices.
Powerheads:
Powerheads are in-tank pumps that hand on the rim of your tank (or by suction cups on the glass). There are lots of different styles and brands of powerheads. Each have their pros and cons. Which ones will be best for your tank is something each aquarist has to decide for him or herself.
Placing powerheads in an aquarium is an imperfect art. The best way to go about it is simply by trial and error. Don't be afraid to get creative. You can point powerheads at walls, rocks or even other powerheads. It's whatever works for your tank. As far as how many you should use, that's going to depend on the size and design of your tank. However, general rule is, if you don't have any other significant source of water flow, use as many powerheads as your aesthetic sense will tolerate. Short of pointing a fire hose into your aquarium, it's quite difficult to have too much water flow in a reef tank.
photos by Gene Schwartz and Mike LaPorte
Note: Many pumps and powerheads (especially the bigger ones) can generate a significant amount of heat. This may or may not create a problem in your tank. There is at least one brand of pump/powerhead with external (out-of-water) components so that they don't heat up the tank water.
External Pumps:
External pumps can get you a lot more power than most any powerhead can. However, they're more expensive, more difficult to set up and require more maintenance. For help setting up and maintaining an external pump system, consult with your more experienced fellow aquarists and your local aquarium store.
Surge Devices:
Surge devices are a way to get good strong,sometimes random, (if multiple surges are used) waterflow with periods of calmer waters between surges. A surge device is basically a container that sits above the main display tank. That container is being filled with a pump,and when the water level within the container reaches a critical height, the water is quickly released into the aquarium causing a nice "surge" of water.
1) The first was designed by Dr Bruce Carlson of the Waikiki Aquarium. Its basically a siphon design, that when the water level within the container rises above the siphon tube, water is quickly siphoned out of the container, and "surges" into the main display. When the water level within the container drops below the siphon tube, the siphon is broke, and the surge stops, and restarts the cycle again.
2) The Second type implements a toilet flapper and float, the basic design of which can be found in Aquarium Corals, a book by Eric Borneman. As water fills the container the float rises and lifts the toilet flapper, releasing the water into the display tank. As the water level in the container drops, the float also drops allowing the toilet flapper to reseal and start the cycle again.The beauty of this design is that they will work well with a very small pump to feed the surge, which will conserve energy whereas the Carlson design will not work as well with such a small pump. Other benefits to this design is that they are less noisy, and produce less bubbles and salt creep when compared to the Carlson.