@Tuexzen earlier I gave a quick response, and then Lynn gave a better one, but I've been thinking this question through and here is a more thorough response
Your question was what is fast draining soil... So most potting soil is designed for plants that want a good amount of water, to decrease how often the caretaker needs to water the soil is made up of things which naturally act as sponges holding water - this is usually called 'organic matter' and is chunks of wood, or peat, or other organic material which absorbs water.
For succulents and plants which don't want a lot of water they require the soil to be amended, with things that don't hold water, usually rock based materials. Pumice is volcanic, light weight rock material, crushed down to be easy to add to soil. In this way there are less items holding water (ie pieces of wood or peat) and the water doesn't stay on the plants roots, which causes rot in most succulent plants.
Hope that helps give a bit more info. If you don't have pumice, using small gravel, or sand can help do the same thing, just not as efficiently
