Ghost is a Swedish band known for their Satanic imagery, unique sound, theatrics and over-the-top concerts, and running lore. Their music can be put into lots of different categories and varies greatly depending on the album, but it is generally described as heavy metal or rock, with recent songs taking on a more rock-pop or "evil ABBA" vibe.
Ghost currently has eight members. Seven of them wear near-identical masks and are referred to as the Nameless Ghouls. These members are responsible for playing various instruments, such as electric guitar, bass, drums, keyboard, ect. Fans have given each one names to distinguish them but I can never keep track of which is which lol
Ghost's eighth member is the frontman, vocalist, songwriter, and overall main dude of the band: Tobias Forge. However, he does not perform as himself; the band is fronted by what is basically an evil pope named Papa Emeritus. With each album, this pope is "killed" and replaced by his descendant, all of which are just Tobias in different masks. There's a pretty good video(link later) explaining Ghost, which has a line that puts it perfectly: "[Tobias] makes a living larping as his ocs and we just let him".
The basic premise behind the band is that it's supposed to be a Satanic Church taking over the world, with each album telling the next part in the story while also criticizing modern day Christianity and religious institustions, as well as other societal problems. Currently, Ghost has 5 albums and 3 EPs. Each album has its own unique sound, themes, and a new Papa Emeritus to go with it.
One important thing to understand is the concept of Satanism itself. The vast majority of Satanists don't sacrifice babies or conduct rituals in the woods -- they're Athiests who take on the label, usually to show their rebellion against the fact that Christianity is an extremely dominant relgion (in America, I can't speak for other countries) to the point that it influences laws, even though not everyone follows it. People also tend to get rubbed the wrong way by people who use this religion specifically as a shield for their regressive and bigoted beliefs. Ghost as a theatrical group takes the concept, stigma, and Christian perception of Satanism to the max for satire, aesthetics, their storyline, and essentially to prove a point.