An herb is a
plant, or the
leaf of a plant, used by people for some purpose. In other words, it's an herb if you know what it is and what it's good for, otherwise it's just a plant, or even a weed. "
Herb" refers to the leafy part of a plant; "
spice" usually means seeds or bark that can be ground up into a powder.
Fresh herbs are great. You can rub them between your fingers and then smell the bruised leaves to discover their distinctive fragrances, and you can add them to your food by the handful. Most dried herbs lose their potency after a few months, so you should always check your herbs before you use them -- give them a sniff. If they're smelling stale or have no discernable fragrance, throw them out.
Most health food stores in the USA have dried herbs (both cooking and medicinal) in bulk, generally Frontier brand, which is pretty good. This is the best way to buy them. The supermarket will charge you an enormous amount of money for ancient, denatured, prepackaged herbs that aren't worth using.
Common uses and their herbs
I know this is only a partial listing. Please let me know if you'd like me to add anything that's missing.