There are many different cultivars of basil varying in taste and usage.
If you are growing basil in the summer, it will go to seed (bolting). This could be caused by little harvesting. Bolting means that the plant will start to produce flowers which means it is concentrating it's energy and sugar in the flowers rather than the leaves, thus causing a bitter taste in the leaves. Within first signs of flowering, pinch off the flowers and the first layers of leaves after the flowers. This will push the plant to make new leaves and branches rather than flowers.
There are two different types of parley, flat leaf and curly. The two taste the same and grow at the same rate. Curly is typically used in cooking with heat and flat would be used in a raw. Harvest the outside stems before the inner stems to keep the plant producing.
Read more about parsley at http://mysticalmagicalherbs.com/2014/06/18/all-about-parsley/