There are certain types of missions.
Mini-missions are single missions without a story or sequals. They were the first type of missions before "Mission Packs". Usually, the users create them if they aren't registered to gtaforums and they just like to create and share their missions.
Originally mission packs were experimental waste of Rockstar Games in San Andreas. They are never used in the original game, but they can be used for modding. Mission packs are used to get additional script content into the game.
Mission Pack is certain series created by a DYOM user and provides plot about the series by creating a thread in DYOM section of gtaforums. Mission packs are usually packed using .zip and .rar, sorted as "Chapters" and uploaded to DYOM Website.
- Main article: Storyline