(Current post is about Oblivion)
Its basically an open-world game, where you can literally do what you want, when you want, if you for example don't want to make the main quest, just dont talk to the NPCs that give them and do everything else in the game, you can join the Dark Brotherhood, an assassins guild where you get to kill people for money; the mages guild where you can do some quests that'll guide you on how to become a great mage; you also have the thieves guild that as the name says, their missions are for you to steal stuff from other people, and a couple more.
You then have houses to buy in each city, the houses can vary from a small hut to giant manors, you can battle in arenas to get money, you can invest in shops if you're a good merchant, you can perform the missions for the Daedra(basically demons) to get unique artifacts, become a vampire and survive by drinking blood and much more
And if you want, all this can be made with either separate chars or the same character, be a bow-wielding arch-mage, be a fiery battlemage by training both magic and combat, be a vampire necromancer and control the undead or become the most powerful invoker in the world by being able to summon tons of monsters, etc.. the possibilities are endless!
This is all without mentioning mods cause if I did, I'd spend the rest of my night here
tl;dr
TES IV: Oblivion is great,
Skyrim will only be better