It seems like these days everyone is contending with some sort of restriction on what they eat, whether self-imposed, doctor-recommended, or something else. It can be hard to know what each one actually means and what people can actually eat. Here's a quick rundown:
Vegetarian: No meat; some vegetarians describe it as "no food with a face." Most vegetarians will eat dairy and eggs; however, marshmallows, jellybeans, gummybears and gelatin are off the table (because gelatin is made from cow hooves -- who knew?). So are many red-colored foods; natural red color is OK but many foods are colored with an additive called carmine, which is made from ground-up beetles.
Vegan: They avoid everything vegetarians do and stick pretty much to plant foods. If you're feeding a vegan, check labels on products that would seem OK: some brands of soy cheese include casein, a milk protein, for example. Super strict vegans also avoid things like sugar, (because of a manufacturing process used in some kinds of white sugar that uses bones), and beer and wine because they are sometimes filtered through, get this, fish bladders. Look for products labeled "vegan" to be sure they are OK.
Gluten-free: This is the latest trendy thing, but lots of people acually have to avoid gluten or they'll become very, very ill. As a general rule, avoid things made with wheat or barley (including anything breaded) -- not just baked goods but beer, malt vinegar, etc. There are more and more gluten-free options available now, but it's wise to ask first what is safe if you are cooking for a friend who follows a gluten-free diet.
Paleo: This is supposedly eating like cavemen ate, but given that Paleolithic people in my area ate mastodon brains, not so much. Not to say it isn't a very healthy way to eat; paleo focuses on unprocessed, whole foods, with no gluten, dairy, or legumes. Alcohol is usuallly forbidden. There are lots of paleo websites out there to give you ideas of what to cook, and there is a surprising amount of delicious food that paleo adherents can eat.
Do you follow a special diet? Which one, and why?