There are generally two different types of bass or guitar tuner, the first type is specifically for either a guitar or bass and usually has a selector switch or button that allows you to choose to tune either instrument. This type of tuner has a fixed range of static tones to tune to, EAD and G on a bass with the addition of B and E for a guitar.
The secondary type is the Chromatic tuner and has a series of 12 notes that can be tuned to, which are basically the tones within the chromatic scale. This gives the user the choice of tuning to any note within this scale and is handy in situations where you may need to play in a non-standard tuning.
The chromatic tuner is also useful for checking intonation or practising on a fretless bass. The most practical tuners will have both an LED read-out and a mechanical needle to watch whilst tuning. This is so that you can tune on a darkened stage - using the LED read-out, and also in bright daylight (where the LED would be impractical) using the mechanical needle.