Knowing what type of climbing hold you're using and the best way to utilize it can greatly increase your climbing skill. Natural rock can facilitate many climbing holds, though the majority fall into a few select categories. Some holds are large enough to fit your whole hand, and others are no bigger than the thickness of a credit card.
A crimp is a small climbing hold that can only fit a part of your fingertips on it. Some crimps are large enough to fit an inch or more of your fingertips, while others are only the thickness of a credit card. You can grip a crimp with a closed hand in which your thumb wraps around your index finger, or with your fingers and palm open. Keep in mind that while gripping a crimp with a closed hand increases your strength on the hold, it places more pressure on your fingers, and could possibly lead to tendon or pulley injuries.
A pinch is exactly what the name indicates, a hold that is gripped with your fingers and thumb as if you're holding a can of soda. Pinches can found at all angles and on all types of rock features. The key to these holds is to squeeze hard.
Slopers are smooth, flat holds that don't have any kind of hole or edge to grab. Slopers can be found on all types of rock, but are more common on sandstone. Holding a sloper is much like palming a basketball, and you use the friction of the rock and your body position to increase your grip on the hold.
An undercling is a hold that faces down the rock face; your hand faces palm-up when you're holding it. Underclings are usually found in caves and on steep climbs. Gripping this type of climbing hold is much like doing a bicep curl. With the right technique and body position, most likely high feet, an undercling produces a large amount of upward motion with a smaller amount of energy used.
Sidepulls are much like a crimp, except you pull sideways instead of downward. Sidepulls can be found on all types of rock, and can be held with an open or closed hand. These holds also produce a lot of upward motion with less effort.
Jugs are large holds that you fit your entire hand in. This term describes some of the biggest and easiest climbing holds that you'll find. Jugs come in all shapes and sizes, use a smaller amount of energy to hold and are better used to rest while climbing. This term comes from the handle that you grab on a jug of milk or water.