TLDR and miscellaneous tips at the end

What is it like?
Cowboy simulator. Smooth animations, a detailed open world, stunning graphics, quality voice acting, and an incredible amount of polish. When I say this, I speak for both story (Red Dead Redemption 2) and online (Red Dead Online/this product). Both "modes" are actually extremely similar in what you can or cannot do. The main difference is that in online, you get to create and control your own protagonist who experience a different story. Who they become is up to you to decide. The entire map is available for exploration and most, if not all guns/horses/food/etc. available in story are also available in online.

What are the players like?
What players? After a few weeks of playing, I can count the number of people I have seen in free roam on my two hands. This can be both good and bad--if you like meeting new people in the game, you're better of using a third party site or platform to find friends. If you like to be immersed or are just weary of griefers, this is a godsend.
But for the people that I do see, what are they like?
GTA Online but most aren't blood-hungry psychopaths or literal Gods. A vast majority of the people you see will probably just ignore you. Tryhards/griefers and modders/cheaters are present, yes, but they are incredibly rare in my experience.

As for the ingame economy?
Cash is the standard currency--it's easy to get and earned from most things in the game. Gold is the "premium" currency of Red Dead Online; it can be used to bypass level restrictions, purchase roles, or be used in place of cash. It's a bit harder to come by compared to regular ol' cash, and is rewarded in smaller amounts. You will need a lot of it. Expect to farm/grind for currency often. Just keep in mind that there's no need to hurry. And don't get burned out. It's also important to note that the way rewards work in this game is a bit weird. In many instances, you get paid MORE cash/gold for taking LONGER to complete a mission or objective. So take your time and enjoy. Don't rush--you'll only sandbag your rewards anyways.

What should I be doing when I start?
When you first start out (after the tutorial missions), you will mostly be going to Horley and doing online's story missions for early cash and a bit of gold. Once you discover more lucrative sources to make bank, you can sort of "farm the way you want" with specialist roles. These "roles" or "jobs" let you earn cash by doing specific activities that are exclusive to said role. Like gunplay and lots of action? Save for a bounty hunter pass. Want to start a hunting business? Become a trader. Like alcohol? Sell moonshine! Are you intrigued by animals? Consider becoming a naturalist. Fond of exploration and/or treasure hunting? Collector. Note that getting into roles requires a steep gold investment and that only the bounty hunter role can earn gold from their activities.

Miscellaneous tips
- I recommend you buy the collector bag to become a collector as soon as possible. With the collector role, you can stock up on treasures found in the open world, then sell them in sets for a large sum of money. Each treasure you collect also gives you experience points to increase your rank. This is especially important because you get a treasure map every 5 ranks that provides you with a lot of gold and cash in addition to your other sources of income. I recommend looking for antique alcohol bottles and the Swords and Pentacles tarot card sets as these collectible sets are easily obtainable without any additional investment aside from the collector bag,
- Speaking of collector, hit shift+tab ingame to toggle the Steam overlay. Select "Web Browser" to open a new web browser tab within Steam overlay and search for "jeanropke map". This will lead you to a site with a virtual Red Dead Online map that updates in real time. With it, you can see the location of every collector item and mark them on your own map ingame. The map also has other important markets for fast travel points, Madam Nazar (collector vendor), legendary animal spawn locations, and some other useful things.
- Aim to upgrade your Carbine repeater to the Lancaster repeater. The Lancaster isn't too cheap but it has a high rate of fire as well as balanced range and accuracy, making it an all-around reliable weapon. Oh, and don't forget to clean your weapons every so often so that their stats are good. To do this, make sure you have gun oil in your inventory (purchased or looted), hover over a weapon and press the scroll wheel/middle click.
- Press L to pull up the player menu. You can do a lot from here but going to "Online options" at the very bottom allows you to change your playstyle between offensive (on by default) and defensive. Defensive mode is good for players who aren't looking to pvp, as it gives you damage resistance from players, disables auto aim lock on, immunity from executions and the lasso, and other buffs that promote pacifism.
- Don't forget to do your daily objectives (accessible from the player menu, L key). These are actually an incredible source of easy gold. Most challenges are pretty easy and there's always at least a few that don't require pvp or interaction with other players. You only need to complete one daily objective per day to start getting gold reward multipliers for them. The multiplier increases every 7 days, up to 28.
- The game seems to start with very low volume in Window's volume mixer. This is probably because the game's starting cutscene (the shotgun) is actually very loud on max volume. Be sure to check/change the game's volume in Windows mixer every time you boot it up or switch sessions.

TLDR
Farm-heavy and (possibly) lonely wild west life simulator with a variety of jobs. Better with friends.