These kinds of games can become very addictive if they gel with you, you are essentially in a never ending grind to get stronger as you progress through a story mode and eventually into endgame activities, all centred around killing monsters and finding/managing your gear, levelling up your skills, which in turn allows you to push higher, get better rewards and so on. They are designed to be time sinks and can potentially offer hundreds of hours of entertainment over their lifespan, it's all about the journey in these games, not necessarily the destination of how you want your character to be.
You have a number of classes to choose from when you start, all very different from one another. There are also two options of World Tier when you start, these basically modify the difficulty and rewards you receive, WT1 essentially being easy mode, WT2 being your medium mode. The other ones are unlocked once you've progressed through the main story and offer a big jump in challenge/rewards. There's no pressure to move up or down, you can do the whole thing in WT1 without issue and move up as and when you're ready. It can be played completely solo or grouping up online (which offers benefits too), there's plenty of activities throughout the world where players can bump into each other and help one another along as well.
Story wise, it's easy to follow but very lore rich so a basic breakdown doesn't do justice to all of the backstory. It's a properly dark, morally grey world where the forces of heaven and hell are stuck in a never ending conflict, often spilling into the mortal realm where you, the hero, have to rise up and fight. If you are interested in the game I'd actually consider checking out some videos recapping the story of the series so far, it will give you a very good idea of how all the world works and who the big characters are.
It's best suited for mouse and keyboard, but is absolutely fine with a controller, they've done a good job of translating it over to a pad.