The B Vitamin complex is a group of water-soluble vitamins needed in very small amounts for various metabolic processes. They help exercise in contributing to the production of energy that is necessary to make your muscles work.
B Vitamins include: Vitamin B1 (Thiamine), Vitamin B2 (Riboflavin),  Vitamin B3,(Niacin), Vitamin B5 (Pantothenic acid), Vitamin B6 (Pyridoxine),  Vitamin B7, also Vitamin H (Biotin), Vitamin B9, (Folic acid), and Vitamin B12 (Cyanocobalamin).
B vitamins are found in whole unprocessed foods. If your diet is rich in whole grains, fruits, vegetables, beans, and low-fat dairy you are probably getting enough B vitamins. One exception might be B-12, which is naturally found in foods coming from animals. Plant foods have no vitamin B12 unless they are fortified. Vegans or strict vegetarians may need to consider adding a B-12 supplement.  Always talk with your doctor before taking any vitamin supplements.
Sources: National Institutes of Health, www.health-fitness.com.au and the National Institutes of Health Office of Dietary Supplements