Bass guitars have similar design features to other types of guitar but scaled up: thicker strings, longer neck and larger body, etc. This allows lower notes to be created when the strings are tuned to a playable tension. They are sometimes categorized as guitars but are also occasionally categorized as a separate instrument. Although there are many variations, the standard bass guitar has four strings tuned EADG, one octave lower than the bottom four strings of a guitar in standard tuning. Although the bass guitar can be played like an oversized guitar, it also draws much inspiration from double basses and the instrument has a vocabulary of playing styles and music all of its own.
Slapping And Popping 
One of the distinguishing features of the bass guitar is the slap style. It is typically distinct to the bass guitar, although it has been used on acoustic guitars by skillful players.
Slapping is accomplished by percussively striking the string - usually E or A on a standard tuned bass - with the left hand side of the thumb (for a right-handed player). This is done towards the neck of the bass. The thumb is then pulled away as quickly as possible, to create a distinct, "fretty" noise.
Popping is accomplished by curling the fingertip of the index or middle finger under the string - usually the D or G string. The string is then plucked to create a similar sound to slapping on the thicker strings. This is, again, performed towards the neck of the bass.
Different Basses 
The "standard" bass is a 4 string bass, tuned EADG (low to high). Other variations of this tuning include DADG and CGCF. These lower tunings are often used in metal and heavier music, as they extend the instrument's range lower. Altering the tuning of a bass to a lower range (or any other fretted instrumented) by reducing string tension can cause problems that new players should be aware of: looser strings are more prone to "fret buzz", in which a string rattles on the fretboard, producing a sound that is usually unwanted. Loosening strings also alters the tension on the neck, which can lead to warping the neck.
To achieve a clear tone on notes lower than standard tuning, a standard 5-string bass adds a low B string, with the bass normally tuned BEADG (low to high). It is also common to restring a 4-string bass as BEAD, leaving off the high G. There exist strings that go even lower in range, however these are typically found only on specialty instruments.
Bass Runs 
Bass runs are particularly nice sounding. For example if one wants to change from a C chord to an Am chord, they could do a nifty bass run.
--C chord-- --Am Chord-- E A D G B E E A D G B E E A D G B E =========== ==> =========== ==> =========== | 3 2 | 1 | | 2 2 | 1 | | | 2 2 1 |