Gravity

Gravity is a measure of how fast a falling character reaches their maximum falling speed. A character with high gravity may not be a fast faller, but reach their top falling speed faster.

The vertical knockback suffered by characters is based on both their weight and gravity with higher-gravity characters taking slightly more knockback; horizontal knockback is unaffected. However, the affect on gravity is minor.

Gravity also affects how high a character is able to jump; two characters with the same jump force will not gain the same height due to the different gravity each character has. Gravity is hardly noticeable in Super Smash Flash due to the physics while it is very noticeable in Super Smash Flash 2 due to physics being closer to how it is in the main Smash Bros. games. Gravity makes it easy to meteor smash any character.

The above is most noticeable in who has high falling speed, but who is not susceptible to combos as other fast fallers while he is easier to KO vertically and his falling speed brings him down when trying to recover.

Trivia

 * The reason for Tails' increase in gravity is due to the devs wanting to make him have a more horizontal recovery then a vertical one.