Currently, there are three “versions” of retail WoW:
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Free/Trial - no cost, but only gets you up to level 20 in the base game (and by “base game” it means everything but the most current expansion).
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Monthly Subscription for the Base Game - $15/month (or less for longer times). That gets you everything in the game except the most current expansion, which is TWW currently. So the monthly sub alone gets you EVERYTHING up to and including Dragonflight (so 9 expansions), but it excludes TWW content, so that’s levels 70-80 and the new continent of Khaz Algar and it’s dungeons and raids.
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Current Expansion. The only additional cost to play beyond the monthly sub content is to buy one of the three versions of TWW (base, heroic, or epic, which are like $50, $70, and $90). But the current expansion goes on sale every so often, so best for you to wait for a sale to get it, unless you are in a rush to push past level 70 up to 80 and into end game content of Khaz Algar.
Keep in mind the sub gets you both retail (e.g., up to Dragonflight with the sub and TWW when you buy that separately) and classic.
Note that classic has multiple versions right now: classic classic, classic Cata, classic season of discovery (SoD), and classic hardcore. You don’t have to buy anything else to play any of the versions of Classic once you have a monthly sub, but there are some DLC type of purchases for Classic Cata that get you mounts, pets, etc.
Some other things to note if you are coming back from Legion that have changed:
Professions are expansion oriented now and are learned and leveled in each expansion separately from the others, which is unlike the old way where you had to level them in order as the skill points kept building on each other. That means for Shadowlands, you need to visit profession trainers in Oribos, the capital/hub for SL to learn professions for SL, and you level them with mats and patterns from SL. There’s no need to level up through the earlier expansions. The only exception to this approach is archaeology, which is like up to skill level 900 now, but it’s been ignored for a few expansions now.
First Aid as a secondary profession has been removed. Tailors make bandages now, and alchemists make the anti-poison stuff.
Professions were majorly overhauled in Dragonflight and are totally different now for both DF and TWW. Best to check out a guide for those, like on Wowhead or something to get a handle on how things work now.
If you are leveling up to 70, I usually suggest folks stick with dual gathering professions of herbalism and mining, as they give XP, helping you to level, and you can sell the stuff on the AH for some gold. Then at level 70 and starting TWW, pick whatever crafting profession you want and a complimentary gathering profession.
Chromie Time allows you to level from completion of a starting zone for a toon to level 70 in a single expansion worth of content, with everything scaling to your level. You no longer level by doing a zone in each expansion in order until you outlevel the content and are forced to move on by the XP debuff. By default, if you are deemed a newcomer and in newcomer chat, you’ll be put into DF by default, but you can change it any time by talking to Chromie outside the Embassy in Org or SW. And you can freely change it whenever you want.
The Auction House is cross faction now, not separate for Alliance and Horde. And commodities like stackable mats (but not gear) are region wide, so prices for ore, herbs, potions, flasks/phials are lumped for all servers in North America or Europe or whatever region you are in. That has greatly changed the market for those items, but unstackable stuff like gear, pets, mounts, etc., are still server specific (or for your server and any merged with it).
Guilds are cross faction and cross server, too.
Warbands are a new feature, with all toons on your account being your warband, and lots of stuff in DF and TWW are warbound, like reputation gains, so all toons on your account contribute to a given faction’s rep, not separate by toon like before. Some items are now designated to be warbound, so you can move them around between your toons, and they become soulbound once equipped by one of your toons. Currencies in the currency tab are transferable between your toons in your warband by clicking on one to open up the interface for the transfer, so things like timewarped badges and so forth. You also have a warbank, in addition to your toons normal bank and reagent bank, and the warbank is shared among all your toons (sort of like an integrated personal guild bank many of us had/have), and you can buy multiple tabs with gold for your warbank.
If you are newcomer chat, be sure to use that for questions and understanding changes since you last played. Guides there are players who volunteered to sign up to help out with answers and advice, and it’s generally very useful and much much less trolling there as compared to trade chat. Blizzard generally monitors that channel more readily and strictly, so there’s less non-helpful stuff there.
Welcome back and have fun!
/moo 
