The clitoris is equivalent to the penis where sexual stimulation is concerned. Because the clitoris is located well above the vagina, sometimes manual stimulation of the clitoris is essential during intercourse to help a woman reach orgasm. The G-spot is an area in the vagina which overlies the paraurethral glands. This area when stimulated can produce orgasm in some women. This actually is stimulating the prostate which can enhance male orgasm.
Masturbation is the stimulation of one's own sexual organs to bring about orgasm.
Some religions have taboos about masturbation and it is best that you follow your own conscience in this regard. From a non-religious point of view, masturbation is a harmless way to release sexual tension without risk of sexually transmitted disease, pregnancy, etc. It is a way for people without partners to release sexual tension and urges. This is a much healthier way to do this than engaging in anonymous or emotionless sex. Masturbation can help you learn about your own body and its sexual responses.