The bit depth just tells how many bits to use to describe a single pixel's transparency. Usually, it's arranged like this:

8-bit: palettized - you choose your own palette but the same palette must be used throughout the entire level but a different palette can be used in a different level provided all objects use it.

16-bit: uses D3D - not recommended for fine or smooth textures and it usually won't come out as expected. Best with rough-looking textures.

24-bit: uses D3D - true color without alpha channel. Best with smooth-looking textures that 16-bit won't display correctly.

32-bit: uses D3D - true color with alpha channel. Use only if you need varying degrees of transparency from the alpha channel.

For the true color types, the first 8 bits are for red, the next 8 bits are for green, the third set of 8 bits is for blue and the final set of 8 bits is for alpha (transparency; the final group only if using TGA files with alpha).

The bit depth has no impact on the size of the quant and pixel, it only gives more options for color and/or transparency.