Trademarks apply to words, symbols, or devices used in trade (that's important -- if you aren't selling/trading something, you can't trademark it, naturally) to indicate the source of the goods or services. The trademark attempts to eliminate confusion between what you and I are selling. If I call my restaurant McDonalds, I invite confusion with other restaurants that coincidentally have the same name. So I would probably fail to receive a trademark, even if my restaurant serves only macrobiotic vegan soups. I could probably trademark the name "McDonalds" for my line of glow-in-the-dark motorized litter boxes, however.
Below is a brief overview of USPTO's trademark resources available on the World Wide Web. Please see the USPTO's trademark sectionfor a complete listing of their services: