Forward Tilt, reffered to as as a "Side Tilt" or a "Strong Side" (commonly abbreviated to "F-tilt"), is a Strong Attack performed by any character by holding the control stick forward and pressing A while on the ground. This is distinct from an Forward Smash Attack, some forward tilts can be "angled" up or down to some extent, pointing the hitbox in a diagonal direction instead of totally forward. Most attacks that are angled up are slightly more powerful than if they weren't angled, while some are weaker if angled downwards. Knockback is also affected by angling, with up angles providing more vertical knockback and down angles providing more horizontal knockback. Strong Sides cannot be charged like Smash Attacks.
The majority of forward tilts are used just for spacing, a majority of them are weak for combos outside of facing a wall. Pikachu, Ness, Donkey Kong, Kirby, Mario, Luigi, Jigglypuff, Falcon, Samus, and Fox are examples of this. Of the more basic forward tilts, Captain Falcon is usually considered to have the best forward tilt while Donkey Kong and Ness are considered to have the worst. Samus' is also slightly unique as it is slightly quicker and is more often used than the other "bad" forward tilts due to Samus having few good ground moves. The other forward tilts are as follows:
Yoshi: Surprising knockback for a tilt move. Used to combo into forward aerials for gimps (whether they be short hop, djc, jump, or full jump. Also can combo into another forward tilt, other aerials, down special move, or up special move. Usually considered the overall best forward tilt in Super Smash Bros..
Link: Higher knockback than most forward tilts with good range but heavier ending lag. Mediocre range but overall a good edgeguarding move.
Ganondorf - Kicks straight forward. Very much like Captain Falcon's, aside from not being able to be angled and stronger with more horizontal knockback. Commonly called the "Sparta Kick" of the film "300", where Leonidas, King of Sparta, kicks the Persian into the hole - however, Ganondorf used the kick in Twilight Princess, which came out before the movie.
Snake - Darts his knee with fixed knockback in front of him and after a powerful hammerblow. Great speed, great priority and great range in (both hits have disjointed range), doing a total of 21% and high knockback, making it arguably the best forward tilt in the game. Great damage racker. First he knees with set knockback against grounded opponents (but has knockback on aerial opponents). If another Forward Tilt is done right after the first one, he'll do a double fist that's stronger than the first one, like Toon Link's Forward Smash. The properties of the first kick can be exploited. Doing that is called Knee Locking. Overall, one of the quickest moves of his arsenal, and is a very powerful kill move at a high percentage.
Olimar - Thrusts his body forward in the air with a kick.