The double bass, also known as string bass, upright bass, contrabass, contrabajo or "big brute of a thing" has the ability to lay down the harmonic foundations for any group. It is the lowest sounding member of the string family, with the disctinction of being tuned in fourths, rather than fifths like a 'cello, viola or violin. The normal double bass has four strings and, like other members of the string family, comes in a range of sizes. Whilst many players actually use a 3/4 size instrument for convenience of travelling around, a full-size is available (typically used in full orchestras where a big sound is required) and smaller versions are available, right down to 1/8 size (or "mini-bass") which is ideal for young players of around 8 to 10 years old, just starting out. Playing bass can become a full-on work-out as there is a lot of finger-board to cover.
Although commonly 4 stringed, the bass now has variants, allowing the player to drop down below the low 'E'. You can find basses with a contra-C extension (which looks like a flute strapped to the scroll at the top). By flipping a lever, an extra length of string can be used, stopping the notes either by finger operated levers, or by moving the hand back. There are also basses with 5 strings, the 5th being tuned a fourth lower, to the B. 5 string bass guitars typically have the same tuning. For those of us without these luxuries, it is sometimes necessary to tune the lowest string down a semitone or even a full tone in order to play the lowest notes required in a piece of music. However, if these are not essential, it's normally much easier to play up an octave.
Unlike music for brass basses (in wind bands and concert bands), double bass music is transposed - writing it an octave higher than the sound. This is a convenient means of putting the music on the staff, rather than relying on a plethora of leger lines hanging below. Occasionally you might see an '8' at the bottom of the clef symbol to clarify this - I've seen it mostly when the copyist was using a computer score notation package, so that when playing back the written music using computer sound fonts, everything sounds right.