Pokemon? Wel... don't really like that, there are a few references to it but they are just references.
It's what could be called a twinstickshooter/roguelike: everytime you start a game, the layout of the level is randomized - there is a big database with rooms and monsters, but the order in which you encounter them is always different. You go thorugh every level, called floor, fighting monsters and collecting items that can help you. Those items can be activated (and you can have just one active item at once), or passive - item that change your stats, that help you in many ways... seriously, there are a lot. And there are bombs to damage enemies, destroy rocks (sometime they contain something) and find secret rooms; keys to open locked rooms which might contains something usefus; and coins to purchase items for the shops.
The first time you play you have to go through a very few level and kill Mom, the first "final boss", After that, the game continue to expand and new levels and stuff unlock while you progress.
There is a lot of variety: enemis, bosses, items, and many items have interesting synergies between them. Every time you play you'll probably see something different.
Be careful altough, it might be very hard at first, but in my opinion it's totally worth it.
Oh and for the hour, I have around 200 hours in the original, and around 700 in the remake...