Mostly revolves around either businesses, heists, or PvP modes.
http://gta.wikia.com/wiki/Category:GTA_Online_Content_Updates
So, PvP modes are essentially your deathmatching, battle royale, CTF/defense sort of things (tag you're it; sheep vs wolves; defend the plane/person/thing), and also various races but especially stunt races now. There's a sort of Hot Wheels mode, a Tron bike mode, but especially various stunt races and stunt track modes kinda like at the top of thread -- stunt races that zoom across a track hovering above the city.
Heists are basically just that, but I say 'heists' to include some of the co-op job missions that are also popular and also involve essentially a team of players, big shootouts, and stealing or destroying a vehicle of some sort. Basically, your co-op shootouts that kinda still take place in the normal open world rather than a deathmatch arena.
Businesses are mostly based around getting items and then selling items. So, you buy a business: CEO office, warehouses, biker club, weed farm or cocaine lockup, weapons bunker, smuggling hangar, etc. Then, you get items: buying coke, stealing coke, stealing crates, stealing cars, or otherwise basically getting X item that usually involves NPCs trying to kill you and taking it back to your business as other players try to kill you. Finally, you deliver the item, and that usually involves trying to drive a poorly/undefended vehicle of some sort from point A to point B before other players kill you (you get marked on the map and they get a small reward for griefing you).
It's the most 'busywork' part of GTAO but kinda the routine way to make money, especially solo, though perfect Heist teams are the best overall way. There's a big RPG part to it, too: you can buy warehouses all over town, different business types (coke, money laundering, meth, etc), buy CEO executive offices, low-end and high-end apartments and houses, biker clubs, etc.
For me it's the main reason I play routinely... it's monotonous gameplay but it's a fun little RP to kinda grow my contraband businesses, buy cooler apartments or more cars, etc. It's repetitive and slow, and kinda MMO/mobi in that regard, but if you ever liked kinda 'doing business stuff' in GTA SA like the trucker missions, the trafficking missions in GTA V, and buying new properties -- it's kinda like that but on steroids. Way more content but way more expensive and monotonous, too. There are a few pretty darn fun modes within it, but expect a lot of driving and praying no one bombs you lol. But it can be cool in a RP or MMO [progression] sort of way if you do it in moderation, and just sort of 'grow your empire.'