What is the "g" spot?
The G-spot is more formally known as the Grafenberg spot, after the guy who described it first in the medical literature. Some folks call it the paraurethral sponge. This structure, whose existence is sometimes debated, is located on the ventral (belly-side) wall of the vagina, about 3cm (an inch and a half) in from the opening. Its texture is different from the rest of the vagina. It feels like a patch about 1 inch across (the size of an American 25¢ coin) with soft ridges. It's called the paraurethral sponge because the tissue is spongy, and it's near the paraurethral glands. The G-spot is more responsive to touch and pressure than the rest of the vagina, and many women find G-spot stimulation highly pleasurable.
For more info on the G-spot and female anatomy, please check out Pink Parts - Female Sexual Anatomy. And if you want to learn more about sexual response, check out Sexual Response & Orgasm: A Users Guide.