Lassen Peak is either the southernmost peak in the Cascade Range or a northern sentinel of the Sierra Nevada--take your pick. Here is it considered part of the Cascade Range, last in a line of tall volcanoes that starts with Mount Baker in Washington.
Lassen Peak was more famous prior to 1980, when the eruption of Mount Saint Helens in Washington robbed it of the distinction of the most recently active volcano in the contiguous United States. Signs of the 1914 eruption of Lassen Peak still linger, and it would surprise no one if it erupted again soon.
The area surrounding the peak is a National Park, and a very steep hiking trail climbs 2500 verical feet up the ash and pumice slopes to the summit crater area. If you don't use this popular trail, the entirely rotten volcanic rock on the mountain makes any other route a dangerous and miserable idea that is very rarely undertaken.