Kirby has a variety of strengths that make him both one of the easiest players in the game for beginners to use as well as a very effective player for veterans.
Puff-Jumping
Kirby has five puff-jumps that make him arguably the easiest player in the game to return to the map with, especially for beginners. His five puffs allow him the greatest flexibility in his return path of any character (except perhaps Pikachu, whose return in unpredictable). By pushing the analog stick down quickly, Kirby can drop down to avoid an attack but still recover with his remaining puffs. When Kirby is knocked a long ways off the map, using his puffs with a second or two in-between the puffs allows for a greater distance to be covered. Also, when knocked far off the stage, Kirby can use his fair drill kick to increase the horizontal distance he can cover between puffs.
Juggling
Kirby can use his utilt to juggle quite effectively. Depending on the weight of the character, he can normally juggle about three to five times (more for heavier characters). This can be followed up with a down drill kick for a very effective combo.
Smashes
Kirby has the fastest recovery from his forward smash attack in the game. This speed can be used to pile up some very effective smash combos, especially on level surfaces or against walls.
His very wide down smash is also very effective for clearing out the area around him and blocking attacks from underneath him.
Quick Punch
Like many other players in the game, Kirby has a quick punch attack that can break players' shields if they remain in range long enough. His quick attack (executed by very quickly button-mashing A) has one of the longest ranges of the quick attacks in the game.
Throws
His grab range is lamentable, but his throw is unique. Kirby flies to the top of the screen, off the screen, and then smashes the player he is holding onto the top-most surface on the way back down. The smash isn't incredibly powerful, but it's impressive to behold. One must be careful, however, that Pikachu's Thunderbolt attack doesn't hit him at the zenith of his throw, or he will fly straight off the map.
Drill Kick
Considered by some to be one of Kirby's cheapest moves, the Drill Kick (A + Down while in the air) acts as a spike kick and propels opponents to their deaths when over open space. Oftentimes the recovery time for opponents after a Drill Kick is long enough for Kirby to execute another one, becoming a combo that can be quite powerful. The Drill Kick lasts a surprisingly long time and can often outlast other opponents' attacks when executed at the same time. The precedence of the Drill Kick, however, due to it not being a smash attack, can often hinder its usefulness. An important quality of the drill is that it can be R-Cancelled. If the player using Kirby taps R repeatedly through the drill, Kirby will finish the drill with no cool-down time and go straight into a grab.
Final Cutter
Considered by some to be the cheapest move in the game, a Final Cutter attack (Up + B) executed over open space results in an instant KO, no matter how much damage the opponent has. The Final Cutter attack can deal damage at three different points: the moment it is executed, at its zenith, and when he lands, shooting out a little projectile in the shape of Kirby with a sword. While executing the attack, however, Kirby is extremely vulnerable until the attack's zenith, where propelling downward occurs. The risk in using this move, is that Kirby must land, if he goes off the edge using this to smash an opponent, he can not recover.
Rock Drop
One of the more unique moves in the game, Kirby's rock drop (Down + B) actually has two uses. If executed while one the ground, Kirby will turn into a rock and be temporarily invincible. Unlike other shields however, there is a substantial recovery time, meaning that your opponent can simply stand there and wait for the move to expire. As a result, it is useful mainly for avoiding high damage attacks like smashes or a fully charged Samus blaster. The second form of this attack is far more useful. If Down + B is pressed while Kirby is airborne, Kirby will again turn into a rock, but this time gravity brings him crashing down, crushing anyone not fast enough to get out of the way. With an average damage of 20% this is Kirby's most powerful non-combo move if done correctly. There are a few perils to this technique that you should remember however. In 1v1 matches your opponent will usually have time to get out of the way, so use this move while they are in the air, on in multiplayer when players are distracted by each other. Also, like his Final Cutter, if you use this move while over a pit or edge you will plummet to your death. There is also a less obvious way to die from this move. Turning into a rock while on an angled surface will cause you to slide slowly down the hill. If you slide over an edge before you can recover, you plummet to a grisly end as well.