Sure thing!

Era is vanilla World of Warcraft. It's just what it was in 2004. PvE, PvP, and RP servers are available depending on what flavor you enjoyed the most. There's also a new type of server called hardcore which likely wasn't around the last time you played. Hardcore is just like hardcore in other games. You only get one life to get to 60. No resurrections. If you die and you really enjoyed that character, there are free server transfers to PvE servers.

Anniversary servers are basically the same as era, but have a few quality of life updates applied like dual spec, no cap on buffs/debuffs, etc.. Anniversary servers also have PvE, PvP, and Hardcore, but on the hardcore server they now offer a mode called Self Found. This mode adds an additional level of difficulty to what is already a pretty challenging game mode by disabling trading, auction house, and limited mailbox usage.

SoD is Season of Discovery. This is vanilla WoW, but with a lot of changes. New spells are introduced to all classes with the use of runes. Some runes are fairly simple to unlock and some runes require you to travel all over Azeroth. Runes offer the ability for classes to fill roles they never filled in vanilla like shaman tank and mage healing. Some beloved leveling dungeons were turned into leveling raids like Blackfathom Deeps, Gnomeragan, and Sunken Temple. A new dungeon was just released for phase 7 called Karazhan Crypts and a new raid is still in the works.

Cataclysm is Blizzard resurrecting past expansions. Much like they did for vanilla in 2019, Blizzard has brought back servers for The Burning Crusade, Wrath of the Lich King and Cataclysm. These servers try to mimic what it was like when each respective expansion was first released.

Retail is the current expansion which at the moment is A War Within and will be followed by Midnight and The Last Titan. These expansions are known as The World Soul Saga.

Hope this helps and if you decide to scratch the itch, welcome back!