Typically, they're considered Power Metal (bardic, epic-fantasy/europagan themes(dragons, wizards, elves, dwarves, trolls, dungeons etc), much more melodic and happy-sounding than other metal types). I'd say they're a bit more aggressive and melancholic than that.
If you wish to check them out, my personal recommendations are:
"Mirror Mirror" (the first one I checked out)
"Valhalla" (preferably this live version: https://m.youtube.com/watch?v=CZ_VFJn2kJM)
"The Bard's Song (In the Forest) amd (The Hobbit)"
"Majesty"
"Wheel of Time"
"Punishment Divine"
"Bright Eyes"
"Time What is Time"
"Otherland"
If it's the albums you're interested in, I'd choose what I called the three Silmarils of Blind Guardian, namely "Imaginations from the Other Side", "Nightfall in Middle-Earth" and "A Night at the Opera"