A trademark is any distinguishing mark that indicates a product comes from you. The whole idea ("trade mark") is that it gives you a way to let people know which products are yours and which are competitors / unrelated / what have you, or, viewed from the mirror perspective, to let customers in the market know that they're buying what they think they're buying. That's why a central test of trademark violation is whether a customer would be likely to be confused between two marks.

They can be anything at all, such as entire phrases ("made from the best stuff on earth"), symbols (like the Apple apple), or particular colors, like the distinctive blue that Mauna Loa uses in their macadamia nut packaging (or the distinctive blue that Tiffany's uses in theirs!).