The horse creates a vibrating sound with his mouth closed, from his vocal cords. The strength and tone of the nicker vary greatly, and will tell you what the horse is saying.
"HELLO, GOOD TO SEE YOU"~ The horse nickers fairly quietly, and moves toward the other horse or person he is nickering to.
"HI, BEAUTIFUL!" or "WOO HOO, YOU'RE SEXY!"~ Usually said by a stallion. This nicker is slightly more intense than the one above, made with mating in mind, and accomodated by shaking the head.
"COME CLOSER"~ This sound is usually made by a mare to her foal. It is much softer and quieter than the "hello" nicker. This nicker is usually accomodated by a nudge from the horse's nose (usually towards her flanks to protect her foal from danger).