Commercially the main varieties are sweet basil and the small more compact milder green basil, but these are not sold by variety.
Choose fresh looking basil with unmarked leaves. Avoid blackened, wilted leaves. Store in a plastic bag in the refrigerator for a few days.
Sweet basil, Thai basil and holy basil are available all year round. Green basil is available in the summer months. Basil should be torn not chopped as the leaves bruise easily. Basil is not generally cooked as the leaves can be become slimy and should be added at the end of cooking or used fresh in salads.