In a nutshell
Piccolo is an unorthodox yet very strong and versatile character who possesses some absurdly powerful tools. Some unique strengths of his include his insanely fast divekick, his 2 sequence j.B with instant overhead potential, his parry, his homing projectile and his launching anti-air. Piccolo also has a strangely high stun value on a majority of his normals which not only gives him access to very easy infinite restun combos, but often times allows him to score stuns on the opponent in the neutral game by simply guessing right a few times in a row. Overall, Piccolo is a unique character with many options at all ranges, despite lacking an invincible attack. Good choice for players who enjoy having diversity in their tool-set.
Close st.Y - Chest level elbow attack. Dead average close standing normal, serviceable but nothing amazing.
Close st.B - A short knee to the face. Likewise, very average normal. Same speed as close st.Y, but doesn't push back as far, making it the better choice for most things.
Far st.Y - Piccolo strikes forward with his claw. Pretty bad range for a far standing normal, despite having decent startup speed. Fairly useless.
Far st.B - A straight, outstretched forward kick. Special cancellable. Very long range and one of Piccolo's best midrange pokes, but what really makes it great is that you can special cancel it into his homing Yuudoh Energy Dan so if it hits an airborne opponent, they will have to deal with perfect projectile oki.
cr.Y - Swift punch to the balls. Combo tool for the most part. This move has an awfully high stun value and can cancel into itself, giving Piccolo a simple stun combo just by hitting cr.Y 3 times followed by cr.B.
cr.B - Every day essential low kick. Hits low. Special cancellable but not normal cancellable. Really good range on this, and being able to combo into Mystic Attacks and Yuudoh Energy Dan gives it added versatility. In the corner at point blank range, it can combo into Shocking Stunner which launches, allowing you to get a full combo.
j.Y - Piccolo performs an air karate chop. Can't be special cancelled. Not very impressive, the only real use for this is as a tick throw tool since it is very easy to whiff it against standing opponents. Hop forwards, press j.Y early, whiff, land, throw.
j.B - A downwards-angled jumping kick. This move can only hit once, but has two hitboxes. The first hitbox is on startup when his knee is out, and the second hitbox is when his leg is outstretched. Because the knee on startup becomes active in 5 frames, this move can hit crouching opponents on the way up, giving Piccolo an instant overhead that can be cancelled into divekick for the knockdown. The knee part of the attack also has a ludicrously high stun value. One of the strongest jumping normals in the game, especially good at close range where the knee just beats all other jumping normals due to it's speed. This move used in conjunction with hop invincibility makes Piccolo incredibly difficult to run a standard, close-range offense against.
High Mystic Attack - Y - Piccolo stretches his arm forward at shoulder level. Similar range to his far st.B, but slower and far worse on block due to not being able to special cancel it. Stick to far st.B.
Low Mystic Attack - B - Piccolo stretches his arm forward at groin level. Hits low. Seems to cover more distance faster than High Mystic Attack. Longer range than cr.B, although pretty bad recovery on whiff, making it dangerous to poke with. Probably better off using it as a combo tool to hitconfirm off of things like far st.B and cr.B.
Mystic Throw - Y or B after High or Low Mystic Attack - Piccolo pulls his opponent back towards him then throws them behind him. Knocks down. Follow up to the Mystic Attacks, use Y for the high version and B for the low version. Since you can't get anything else off of Mystic Attack, you might as well buffer this every time you use it; that is, unless you don't want to lose the corner advantage.
Regeneration Uppercut - Y - Piccolo guards his face with his forearm. Upon being hit with a physical attack, his arm is chopped off, regenerated, then used to smack his opponent. Knocks down. Will parry mid and high physical attacks, but will lose to lows and projectiles. This move has 10 frames of startup which is faster than a lot of close standing normals, making it a solid guard counter option against Vegeta, Piccolo, Kid Buu and Fat Buu. Every other character's fastest close standing normal chained into itself on block will beat this when used as a guard counter. It's also a solid anti-air if you can time it right.
Bukuuh Kyaku - B while in the air - The divekick of divekicks. This thing is ridiculously fast on startup and recovers almost instantly upon landing. Uses of this divekick include: "triangle jump" style mobility with a hitbox attached to it, on reaction to jumps, on reaction to hops, on reaction to projectiles, specials and normals, as an air-to-air, as a follow up to his j.B instant overhead, as a fairly safe corner escape... there's really not much it can't do. To top it all off, it's completely safe on block. Absolutely disgusting move. Use it; use it a lot.
Shocking Stunner - A - Piccolo zaps his opponent with MIND POWERS directed diagonally upwards. Launches the opponent on hit, allowing for 2 followup attacks, since the attack itself counts as an aerial hit even if performed on a grounded opponent. Strong anti-air, not for the speed or range, but for being one of the few anti-airs that can convert into a combo. The active frames are quite good too, and the input is very easy, so not a bad anti-air although not one to use on reaction to hops.
Yuudoh Energy Dan - A - Piccolo fires a ball of energy that tracks the opponent. Level 1 projectile. Costs 5 health to perform. One of the best projectiles in the game and a very strong midrange weapon. It's fast on startup with decent recovery, can be cancelled into from Piccolo's far st.B, and it attempts to track the opponent although it will never stop moving forwards, so it can be jumped or hopped over like regular projectiles. Will lose to all projectiles above level 1.
Gekiretsudokan - A - Piccolo throws a powerful energy blast diagonally up-forward. Generic anti-air projectile attack. Costs 10 health to perform. Does its job for the most part, too slow to use on reaction to hops but is still useful as a preemptive anti-air or on reaction to full jumps.
Special Beam Cannon - A - Signature finger laser that once killed a guy. Chargeable projectile attack that most characters have in some form. Costs 10 health to perform. Hold down A while performing in order to charge it up to a maximum of 3 power values. No charge will quickly release a level 2 projectile. Medium charge will release a level 3 projectile. Maximum charge will release a highly damaging level 4 projectile. This attack is unique in that it hits twice, but doesn't knock down and has slightly slower startup than other "Kamehameha" type attacks. Not particularly useful for Piccolo, but it is his only projectile above level 1.
Desperation Attack: Kakusan Yuudouko Dan Y - Piccolo strikes and grabs his opponent, launching them for an air combo ending in that thing he did on the show that one time. One of the less useful Desperation attacks in the game, but it does have incredibly fast startup, making it a strong guard counter option if you can pull it off. Can be comboed into from any launcher or air juggle.