Drums were the original musical instrument, dating back to at least 6000 B.C. Rhythm and melody are the two primary features in any piece of music, and drums function as a purely rhythmical medium unlike any other instrument. The modern drum set did not exist until the 19th century, with the addition of pedals to allow a percussionist to manipulate cymbals or drums with his feet. Differential association is the thought process by which a drummer is able to seperate the action of their limbs to independently perform complicated time phrases without the whole rub your belly and pat your head effect.
The basic parts of the full drum set are as follows: Bass, Snare, Toms, Hi-Hat, Crash, and Ride. Your two feet operate pedals that either strike the bass drum or toggle the hi-hat, while your hands are free to roam over any other drums or cymbals. The bass and snare drums act as the foundation for any beat. The bass drum is the largest piece of the kit, and displaces the most air when struck, making it the most 'felt' piece on any drummers set. The snare drum, a throwback to military drums of old, is so called as it has metallic wire snares on the underside that give it the distinctive "PSSHH!" sound. Toms are pitched drums used for drum fills (mini-solos that act as breaks when playing straight time). Toms are typically found mounted on top of the bass drum on brackets, and to the right of the set on the floor (Floor toms). Cymbals come in all shapes and sizes. The hi-hat is stationed to the left of the kit, and is opened up or down with the foot to change the timbre of sound when played. The crash cymbal is used to accent key changes in the song's structure with a punctuating explosion. The ride cymbal is the largest, found on the right hand side above the toms. It's primary use is for variation from hi-hat time, and adds a delayed ring and jazzy feel.
Although the parts that comprise a kit are similar, people's drum sets are rarely the same. Amateur drummers will have to bear horrible "Frankenkits" made up of whatever they can duct-tape together. Professional drummers will customize their kits to suit either their own playing style or the performance scenario they're required for. Famous jazz drummers like Gene Krupa or Buddy Rich often embraced a minimalist setup, with little more than a bass, snare, and cymbals. Sensationalist modern drummers like Carter Beauford, Neil Peart, Mike Portnoy, or the 15-yr old prodigy Tony Royster Jr. use immense sets with countless toms and cymbals. Keith Moon, Ginger Baker and Billy Cobham chose to forego the hi-hat and use two bass drums instead for added power. Left-handed drummers (those sinister bastards) will set up their drums the opposite way. Setups for drumkits can be created from any combination of infinite variety, but it takes a real drummer to get a good sound out of whatever he/she has. Musicality extends into the rhythm section as well, believe it or not. And enough with the drummer jokes- pianists are just as funny.