Food can be found as loot, purchased, or crafted through Cooking.
If crafted food is eaten for less than 10 seconds, then it will only cause the immediate mana and/or health increase. But if one stays seated for 10 seconds or more, then crafted food grants additional long term bonuses, usually for 60 minutes. A timer in the top-left corner shows how long you have been eating.
Food bonuses do not stack with each other. They all grant the Nourished buff and a new food bonus will replace any existing one.