A double bass is an instrument of the string family larger than the cello - in fact, around six feet tall. It is also known by some by other monikers: American useage sometimes favours the term 'string bass'1, and it has also been referred to as a 'stand-up' or 'upright' bass. You can make sound by using the bow like with any other stringed instrument (called 'arco') or pluck the strings with your fingers (pizzicato or 'pizz'). Bass music is written almost exclusively in bass clef, the lowest of the four commonly used clefs for musical notation.
A common misconception is that the term 'double bass' refers to a bass guitar. Even though both instruments' strings are tuned to the same notes: E, A, D and G, the bass guitar is a different instrument and you should probably not attempt to play it with a bow.
Here are some things that might be worth remembering about the double bass or any other stringed instrument:
If you bow too close to the wooden bridge, the bass will make very strange noises.
The bridge of the instrument is only held onto it with pressure between the strings and the body of the instrument.
The part of the bow where you put your hand and that has the knob with the screw for loosening/tightening the bowhair is called a 'frog'.
It is generally not recommended that a bass be left outside on a very hot day, as the heat can cause the glue holding the bass together to melt like butter. Goodbye, expensive instrument! Also, even though the bass can arguably be considered a portable instrument, since you can carry it with you if you want, it has been the experience of this Researcher that it is necessary to develop a trick for negotiating doorways with a bass, and fitting it into a compact car for transport is a bit like solving a puzzle.
You could open a window in the back of the car and then let part of the bass stick out of the car2. Or the other way, which requires more spatial coordination, is to lower the front passenger seat until it's horizontal, strap the instrument down with a seatbelt or two, and wedge it (gently) into the now-quite-a-bit-more-crowded passenger seats, diagonally.
It is also worth noting that taking a bass upstairs with you while wearing sandals or in a crowded elevator is exceptionally tricky. If you have to travel by aeroplane with your bass, you should buy an extra ticket for it or have special arrangements made, so that you can have the instrument take up a seat next to you It would probably get totally smashed in the cargo section, since wooden instruments are very fragile. They tend not to be waterproof either, so one might be led to suspect that the wood itself would warp if it absorbed too much moisture from being rained on.
There are some modified versions of the string bass that are slightly different from the standard version discussed above. These include the contra-bass, where there is a fifth string (C) added on to the instrument. Another variation is a special type of device with lots of extra strings on it that can be attached and set up on the bass to make it sound kind of like a sitar. There are also electric upright basses, some of which omit the large body required on an acoustic instrument. There are also bass pickups: one end clips onto the bridge of the bass and one end plugs into an amplifier - particularly helpful when playing with a noisy band. Otherwise, the sound is only coming out of the f-holes - the seemingly decorative holes on the front of the instrument which let out the sound made from the bass's vibration.