A trademark is any word, slogan, symbol, design, or combination of these things, that identifies the source of your goods and services and distinguishes them from the goods and services of another. A trademark is essentially a brand for goods and services.
Consider for example, Coca-Cola and Pepsi. When you see each mark on a bottle, you immediately know the source of the beverage. The mark on the bottle prevents you from grabbing a Coke when you really want a Pepsi and vice versa. The name Coca-Cola is a trademark in itself, but when displayed in a logo, the wording in that stylized form is also a separate trademark.
A trademark can be a word, slogan, symbol, design, or combination of these things, as well as a sound (think of the MGM lion’s roar), a color (think UPS brown), or even a smell (like PLAY-DOH’s scent). Pretty much anything can be a trademark or service mark, so long as it identifies the source of goods and services and distinguishes them from the goods and services of others.