I love mustard. It comes in a wide variety of types. Enough so that you can eat it with just about anything. It's pretty much the perfect condiment. You should always have at least one kind of mustard (probably plain yellow), but expanding out into some different kinds is a great idea. Here are some of the basic types that I like to keep in the fridge:
Plain Yellow American Mustard- It's mild and bright yellow in color. Perfect for hot dogs and hamburgers. You can put a lot on things and not be overwhelmed. My personal favorite kind is Plochman's, but the most popular is French's.
Dijon Mustard- This is more of a yellow/brown color and has a tangier taste than the plain yellow. It's the classic French way of preparing mustard, with a little wine mixed in there. It's great with roast beef and on sandwiches. My personal preference (at least as far as those commonly available) is a French brand Maille, which is still sold in most grocery stores. The same company also makes the american line, Grey Poupon, so that's pretty good too.
Honey/Honey Dijon Mustard- This is pretty much dijon mustard with the addition of some honey to make it sweeter. Perfect on sandwiches or with chicken. I particularly like it on a turkey sandwich. My favorite here is the Grey Poupon brand.
English Mustard- It's bright yellow, but it's pretty hot so you only use it sparingly. This is the perfect roast beef mustard. Perfect in small amounts. The standard is Colman's.
Whole Ground Mustard- This has the whole mustard seeds in it. More of a grainy texture with a stronger vinegar taste. I like to just put in on crackers with cheese. But it's good with most anything. Right now I'm going through a bottle of French stuff from Sur La Table, but most makers have this type.
Chinese Mustard- It's pretty mild in taste with a very light brown color. It's really easy to prepare (being just the ground mustard and water) so you can actually make this at home using just the powder. It's usually a dipping sauce for various fried or stir fried items. I haven't found a particular brand that I prefer yet. They seem to be fairly similar. There are also pre-prepared kinds.
Those are just some of the basics. There are hundreds of other varietys and variations. With different igredients added, or ground to different textures. Experiment and try them out. You'll probably find something you like.