Both types of quests are compiled in your Pip-Boy, and must be earned either by speaking to the right people or going to the right location. Though Fallout: New Vegas is an open-ended game that stresses exploration and going about your business in whatever fashion you'd like, the fact still remains that to progress the game's main storyline, as well as all ancillary plots, you'll need to undertake Fallout: New Vegas' many main quests and side quests.
