Jobs are completable tasks issued by factions. Completing jobs grant you K-Marks, Faction Points and Scrips. For missions within Faction Campaigns see Missions.
Korolev
Korolev-issued jobs generally involve mining and industrial processes.
ICA
ICA-issued jobs are fairly generic and involve a variety of tasks.
Osiris
Osiris-issued jobs generally involve flora- and fauna-related things. This includes gathering flora such as Brightcap Mushrooms, or killing fauna such as Striders.