Osu (stylized as osu!) is a rhythm game... Developed by an Australian group, wiki says it first released in 2007. Probably by far the most popular rhythm game out there, and probably the only one that can rival DDR (Dance Dance Revolution) and DDR-clones/lookalikes. And unlike DDR which requires a ridiculously expensive setup (a reliable DDR pad would cost close to $1k and is extremely loud) or being a regular at your local Japanese arcade to play, osu can be played by anyone with a PC, a mouse/trackpad, and a lot of hopes & dreams

Osu was inspired by the Nintendo Ouendan series on the NDS; in that game you use the little pen provided by NDS to click circles/drag sliders/etc on the bottom screen; obviously works well with the NDS form factor. The osu team decided to translate this into PC gameplay where you need to control stuff with keyboard/mouse... and somehow it worked quite well!

Since osu is completely free (I believe it is still very much free-to-play, no idea how they monetize), relatively accessible (see counter-example of DDR above), and is a legitimate & very serious rhythm game, I think it quickly gained a sizable and very passionate player base. And unlike lots of other rhythm games where the charts are curated by a company, osu's charts are created by players & "peer-reviewed" by mods, so there are a LOT of charts, basically any anime/game-related song you could think of is in the game as an approved chart, which further helps grow the popularity. Needless to say it just kept growing from there... I think even back when it was the 2010s and I was playing the game actively, there were already a bunch of community groups, and ppl literally had names for different play styles. I think my style of primarily using mouse but mashing keyboard Z/X key for combos was called the Seiiryu (blue dragon) style or something... I forgot sorry

As for the gameplay itself... Osu's gameplay is actually quite unique in terms of rhythm games especially back then. Back then the gold standard of rhythm games I believe are DDR and IIDX, both of which are vertical fixed-screen drop-down notes where you have to time the fixed buttons to the notes. Osu on the other hand has dynamic notes where circles fly all over the screen. However, this also means that at higher level gameplay, osu relies less on your "sense of rhythm" and more on... precise mouse movements, almost like an FPS. I think nowadays games like maimai/WACCA/Chrono Circle might be similar to osu's playstyle. They did add more game modes though; they have a taiko clone, a "catch the fruit" game which is even more unique than their base game, and a djmax/iidx clone.

And... yeah. In short I think osu could be seen as the gateway drug into rhythm games due to it being free, having charts for just about any song you could think of, and having a passionate community. Now that you've sunk yourself in the rabbit hole, grab your wallet and pay for that $1000 DDR setup you have always wanted, $2000 maimai ADX controller setup, and mortgage on the suburban single-family home to play it in so you don't get complaints from neighbors. You know you want it. Do it. DO IT (/s obviously)