The most common style of pannier rack is fitted to the rear of the bike – but you can also opt for a rack at the front too. If you're carrying a moderate load it's probably easier to go with the rear.
If you're taking a lot of kit, opting for both and distributing it so that the greatest weight is at the back but there's some at the front will improve handling. It also makes the bike easier to push around and carry and less front-light if the rear is loaded so that the weight is close to or aft of the rear wheel.
Just having a front rack and panniers may affect your bike's steering, so should be avoided.