Slamming is putting the ball into the basket. Some acrobatic moves can be added, such as moving the the hands with ball while you on air, or moving legs etc. There are too ways of slamming: at a slam dunk contest (no defence, no travelling) or at a competition (with defence and travelling). Both of the ways are effective.
It is possible to slam with one or both hands. If you want to slam with one hand, you often need to carry the ball strongly. If you slam with two hands, you need more height or a higher jump. One-handed slam is more effective, because it is easier to add some acrobatic moves. Two-handed slams are not so effective, but more reliable.
EXAMPLES:
One-handed: Kobe Bryant, Shawn Kemp, Michael Jordan.
Two handed: Vin Baker, Zydrunas Ilgauskas, Dominique Wilkins.
Slam Dunk Contests are competitions of slaming. NBA Slam Dunk Contests are the most interesting. Kobe Bryant won the Slam Dunk Contest '97 and the best slammer in Europe could be Conrad McRea, Marco Milic or Darius Sirtautas.
Women can't slam so as men do. They have different body structure. Their jump is not high. A woman must be very high if she wanted to slam. There are several in WNBA that can do it, but just a few.
Spud Web is about 175cm(5'10) and he won the NBA Slam Dunk Contest. The minimum height for men is ~165-170cm(5'6-5'8). (The basket is at 305cm (10'2)).