I am asked this question a lot and usually away from earshot of others for fear that they may be asking what everyone else already knows. That is not the case because the person beside you probably asked me the same question when you were not in earshot. Here is some light reading from the definition to features.
Great question! There are so many great choices for your patio substrate. Your choice needs to consider your budget, practicality and preference.
Here are the most common choices with some considerations to help you with that decision.