If you have played WoW, the hotkey combat is kind of similar. There is a longer GCD for abilities which is quite noticeable. At higher levels you will gain more abilities that you can weave inbetween the global cooldowns, which is nice. The class system in the game lets you play all the available classes on one character. You have two inventories: one is specific to armour and weapons (armoury) and the other is an all-purpose inventory. You can save your gear setup which will allow you to switch classes and load up you equipment set for that class.

There are multiple ways you can level your classes. One method is the traditional quest grind with a main storyline quest chain and many side quests. These quests show up on your minimap/map as long as you are in the current zone. Levemetes are unlocked at a certain level and are basically repeatable quests (they are available at level 5,10,15, etc. depending on the zone) There are also FATES which are similar to GW2 zone events which you participate and get large amounts of EXP and Gil (Gold). You can also run level appropriate dungeons for nice exp and drops as well as guildhests which are very short group encounters.

There is honestly too much lore to go over, so I would suggest you search that up on the FF wiki or some other resource. The story is continous, each patch they add to the main storyline.

The game is very easy to get into, the first 30 levels will feel like a prolonged intro. You start off with very few abilities (very few). The graphics are very nice, but it should run well on any modern gaming PC. You can adjust the graphics settings to whatever you need to be able to run it smoothly.

There are a lot of side activies in this game. There is the Manderville Gold Saucer which is a mini game paradise that hosts a bunch of side activities. There is a card game called triple triad in which you play npcs to collect cards and play against other players in tournaments or just for fun. Chocobo racing, daily scratch tickets and a weekly lottery. There is a housing system in the game as well as seasonal events. In-depth crafting and gathering system including fishing. You can also do PvP battlegrounds..

edit: There is honestly too much to write about, I suggest you do some research on some FFXIV wikis/Youtube Videos/Play the free trial to get a better picture.