It's free to play (with a subscription for an XP boost)
It's kinda hard to explain, but you make a crew which levels up like most any other online games. Leveling unlocks different ships/guns/torpedo's/etc.. depending on your crew.
You start in a small neutral frigate and work your way through destroyers, light and heavy cruisers. And eventually into a battleship, carrier, or sub. You can make multiple crews but for the most part once you select a path your either stuck down that path or have to make another crew.
Theirs 5 nations, each with different strengths and weaknesses. USA, Germany, England, Japan, France, and the newly added Russians.
Graphically, its not the prettiest thing to look at, but you can see the different turrets and such that you add onto your ship.
Gameplay wise, while in anything besides a CV or SS you have 2 options to aiming your weapons. Either with an "Auto" FCS where its click and shoot. Or a more accurate but harder to use "Manual" FCS where you aim using your keyboard.
Squadrons can be formed, which then can be linked into a fleet. Their are official fleet battles with rankings on the website. And also a harbor assault mode where a fleet can claim a specific nations harbor for access to unique ships.
Like I said its hard to explain, but its a great time waster. That being said, right now's a great time to get in as alot of people started new crews to level up in a russian ship. So theirs alot of other destroyers and light cruiser out there that you stand a chance against.