It's a pretty cool little gem of an indie game where you play a 'Knight' that has escape-pod'd onto an unknown planet called the Cradle after their community spaceship went down on the same planet. You and all the other players are tasked with travelling through ~30 levels of dungeons DOWN to the centre of the planet - the Core - to find fuel for the spaceship.
Gameplay goes like this: you go to the Arcade and choose a lift to go down and start a journey through a randomized set of 30 dungeons with breaks every so often. You can do it solo or with up to 3 other players. You kill monsters and get loot, called 'shinies' ingame (though this is less of a technical term and more of a fun word to use). Shinies or loot comprises of a few things including money, experience, materials for crafting and Tokens to trade with. Sometimes actual equipment is dropped as well.

But the best things about Spiral Knights are:
A) many kinds equipment and thus character customisation,
B) random dungeon sets to go down
C) bosses at the END of a dungeon
D) equipment crafting and upgrading (also Unique weapons)
E) guilds, trading, friends, auctions
F) simple controls (three mouse buttons + moving the mouse is the minimum)
G) achievements (and 2 TF2 hats :3)

... and the best of all is that you can trade what's called Crystal Energy with other players for money. Why is this so good? Well, first off Crystal Energy and Mist Energy are the basis of going to the next level of a dungeon, so it's important to keep going, and secondly CRYSTAL ENERGY IS A PAID-TO-GET RESOURCE. Meaning that people pay REAL MONEY to get the stuff, and (if you have enough) you can buy it from those people and SELL IT BACK FOR MORE MONEY. Think Stock Markets and the Stock is this Crystal Energy.

Basically, you can play this game FOR FREE and get EVERYTHING. Which is awesome.

However, there is a drawback about this Mist Energy: you start with 100 of it and it costs 10 to go down a lift to the next dungeon level. (There are specifics involved with paying for other's lifts but I'll spare those.) The thing is, you only get 100 per day and you cant buy it with game money. It does recharge but it takes about 13 minutes to get 1 energy and exactly 22hours for a complete refill. You can imagine what this means - less game time per day. Also a con is that crafting costs varying amounts of energy - starting at 10, then 30 and so on - for each time you craft something.

So if you want to play it ALL DAY then you either need real money to buy Crystal Energy (which doesnt recharge and is seperate from Mist Energy) OR you need enough game money to buy Crystal Energy.

Also a con is that it even though it has a shallow learning curve, it is hard to actually get some items and/or equipment.

All in all it's a very addicting game but can only be played for so long per day with a 'low level' character for free.