Basil grows in warm climates, and will flourish here only if planted in sunny places. Basil grown in Italy has more flavour, but is rather fragile. It is sold with the root attached, often wrapped in wet newspaper to prevent it from wilting.
To have a constant supply of fresh basil, try to grow your own in a warm, sunny protected place, picking the leaves before the flowers appear. Cutting off the flower spike will encourage the growth of new side leaves.
There are many varieties of basil, and more seem to appear every day. We grow at least two varieties of purple basil, common basil and lemon basil, and last year we were given seeds of giant basil with its lettuce-like leaves. The tiny-leaf bush basil has a different flavour and grows very easily. We include mixed basil leaves along with other herbs when making summer salads.
Thick leaves from the bottom of the plant stem usually taste too aniseedy and they go black when chopped or pounded. The fragrant smaller leaves which tear or pound easily, will give you the best results, particularly when making pesto.