At the Farmers' Market: Basil
While there's a wide gap between the taste of fresh and dried for any herb, the difference between fresh and dried basil is huge; fresh has a floral, anise-y taste and dried tastes like bitter dust.
There are several kinds of basil; the Italian sweet basil, which is the one I use most often, has soft, shiny leaves with sort of pleated leaves. Thai basil, which is much spicier, has pointier leaves and crevices rather than pleats or ruffles. Holy basil, which is actually botanically a different species, doesn't look at all like either Thai or sweet basil; it looks like miniature oak leaves.
All the parts of a basil plant are edible, though woody stems will be unappealing and the roots are bitter and suitably only for making amari (digestifs). The flowers taste stronger than the leaves, and you will notice the flavor of the leaves gets stronger when the plant bolts (i.e., goes to flower). The seeds are crunchy and edible; you may know them as the little tiny dots in bowls of Persian ice cream.
When picking basil at the market, look for sprightly plants and fresh cuts; bunches with flowers will be stronger-tasting (even the leaves). If you see any sliminess, skip the entire bunch: it will be slimy within hours in the refrigerator.
If you need to keep the basil longer than a day or two, remove any rubber bands or other fasteners, moisten some paper toweling, roll the basil in the paper towel, and put into a sealed plastic bag in the refrigerator. You'll get another few days out of them, albeit at a lower quality.
To create the thin ribbons of basil used as garnish in restaurants, stack five or six basil leaves on top of each other, roll like a cigar, and cut crosswise.
Basil makes wonderful pesto (try making it with a mortar and pestle; you will be amazed at the difference it makes); young, tender leaves dropped into Thai-style soups will change the entire character of the dish. Basil makes a wonderful addition to panir-e sabzi; buzz some good feta, garlic and olive oil in a blender to make a thick spread, then spread on flatbread and top with basil and other leaves (cress and tarragon, especially) and crunchy vegetables such as cucumbers, radishes and jicama.