Herbs intrigued me, especially after reading an article in Runner's World. Dabbling in them got more serious. More books got read. More herbs were added to the garden.
Herbs are not a single class of plants, it's more a general theme of what can be done with these plants. Some herbs are relatives of carrots (parsley, cilantro), some are related to a lot of other herbs (mint, oregano, sage, rosemary). Some are related to flowers (yarrow, foxglove). They have their own personalities and needs, making them unique and interesting.
Some herbs make good additions to food adding flavour, most herbs have some sort of medicinal content, even if it is very small.
Arugula, basil, bee balm, chives, cilantro, dill, fennel, horseradish, lemon balm, lovage, mint, oregano, sage, tarragon, thyme.
Anise-hyssop, catnip, echinacea, horehound, lavender, maralroot, marshmallow, meadowsweet, rue, sorrel, wormwood, valerian, yarrow.
There is no generally excepted definition of what makes a herb difference from a spice. Both can be used to impart flavour in a cooked dish. My working understanding is that a herb is a plant or part of a plant that is used fresh, dried or frozen. Whereas a spice is a plant or part of a plant that is processed in some way to bring out its desired properties.