The three main categories of weapons are spewers, charge guns, and rollers. Spewers are your standard weapons, from explosive paint ball blasts to rapid-fire sprayers, and cover the field from mid to long range. Charge guns are the sniper and single shot weapons, and the longer you charge them the further they go, but they fire fairly slow and your movement is limited while they charge. Rollers are the biggest gameplay change, and allow you to continually spread paint as you run around, in addition to unleashing a splattering wave of ink when slammed against the ground. On top of this, you have a variety of secondary grenade-like weapons from mines, sticking bombs, and portable sprayer walls to echolocating markers that pick out enemies for you and your team. Finally, each weapon has a super-charged power that does something like call in an ink strike from above, give you a super ink bazooka, or protect you and nearby teammates from all damage. Finally, the clothing you unlock and use to customize your character's appearance has a variety of buffs on it: Fast respawn time, faster movement, higher defense, or even stealthed movement in your speedy squid form. These buffs have a single fixed slot and a handful of random ones that unlock the longer you wear the clothes - though you can re-roll those slots by playing to slowly craft the perfect item. It's a nice mix of traditional shooter game leveling up and randomized goofiness - it might bother some players, but I didn't mind it at all as it encouraged me to try out new weapons and playstyles based on what I got.
The multiplayer gaming is fun, and evenly balanced games are fast, furious affairs that seem much longer than the handful of minutes they actually last. Sadly, if your team is bad and you're getting steamrolled... the same is true. The nastiest superweapons tend to become a bit of a runaway leader mechanism, as only when you're spreading paint do they charge, so a team of more skilled opponents can lock down the entire match just by taking what ground you have, charging up their big attacks, and splattering your whole team in one go over and over. Thankfully, there's not too many maps where this is possible - the symmetrical design helps you out with a variety of useful paths to get around - and the map selection rotates every four hours. The six maps the game is launching with are quite fun, though some of the more open ones are very challenging for new players to understand and master. They're available in sets of two which rotate every four hours, which adds some spice and keeps maps fresh but can be boring for binge-sessions of the game.
That's the basics. If you're looking for an in detail look at the game modes, I recommend the very detailed Splatoon Nintendo Direct.
The game's single player is surprisingly in-depth for a title that has been marketed as a multiplayer game, combining elements of platforming and shooting in a style of game that reminds me of Ratchet & Clank or Jak & Daxter. While I don't want to spoil too much, splattering down octopus-like bad guys and leaping from platform to platform on ink trails doesn't really get old - and the level design manages to keep some of the mechanics fresh by continually using them in new ways. Despite the good design, there's no truly great design. I'd hoped for more puzzle segments and clever mechanics subversion, and there's opportunity for Portal-like complexity that goes unfulfilled. Hopefully some of Nintendo's DLC can fill that hole. There's also some impetus for single player even for multiplayer devotees, as unlockables and secrets from there can transfer over to multiplayer matches for new weapons and tricks. Ultimately, however, multiplayer is the main event - even if singleplayer is nice - and if nobody in the house is going to try much multiplayer you might want to rent or skip the game.
Bottom Line: It shouldn't be as good as it is... but who am I to deny something this inkredible?
Recommendation: Adding another casually enjoyable multiplayer game to your Wii U's lineup will serve you well, but try before you buy if you're only interested in singleplayer.