Oregon is home to numerous Volcanoes, and while they are all in slumber, they have the potential to be quite dangerous, but that doesn't stop tourist and travlers alike from visiting some of these.
All of Oregon's volcanoes lie in the Cascade mountain range as can be seen on the map below. There are four major volcanoes, all of which are dormant. The last volcanic activity in Oregon was in 1955 (Mount Jefferson)
Basically, Crater Lake is a lake nestled in a crater on a dormant volcano. What makes lakes like Crater Lake so rare are the way Caldera Lakes are formed. Geologists suggest that Mount Mazama was a composite volcano (A steep volcano comprised of many layers of lava flows) the top literally sank into the top after an eruption after time, the crater filled with water.