Drums are percussive instruments built by stretching a membrane across an opening in a vessel. The sound originates from the vibration caused when you strike the membrane, called the head, with another object. Drumheads can be made from the skin of a goat, cow, antelope, or sheep, or even a synthetic material. Across the world, cultures carve drums out of wood, sculpt them out of clay, hammer them out of metal, or mold them out of plastic. The kinds of drums that exist vary as infinitely as the people who use them for communication, ritual, or entertainment.
A common, easy way to classify drums types is by their shape. Most drums fall into the following categories: cylindrical, barrel, conga, waisted, goblet, or bowl. A frame drum, with its squat hoop, and a long drum, which is thin and tall, are both the cylindrical variety. A barrel, such as a Tabla, goblet, such as a Djembe, and bowl, such as a Nakari, are named for their shapes so they are easily recognized. The conga tapers at its bottom, and the waisted drum tapers in the middle, as with a Changko's cinched waist.
Even among these shapes, a drum might have a head at either end of the body or only on one. Congas and bowls have one head, but tambourines (frame drums) may have two. Also to consider is whether you can hold the drum off the ground by cradling it under your arm or setting it on your lap. Some drums can be carried while walking or dancing while some drums are stationary, such as the Djembe from Mali.
One characteristic all drums share is their pitch. Drums have a certain tone due to the body shape and head size, but not all drums are specifically tuned. A "pitched" drum's pitch can be adjusted by pulling a cord or moving a peg to stretch the head. Several drums that are part of a set, each with a pitch tuned to harmonize with the others, would also be considered pitched percussion, such as Bongos. In the Western world, we are familiar with non-pitched drums, such as a bass or snare drum in a drum-set. These can be used in combination with any harmony or key.