As a general best practice, every Gift entered should have both a Project and a Segment. These values allow for clear tracking of the success of various fundraising initiatives.
Segments are one part of the Campaign structure, which consists of Campaigns, Campaign Communications, and Segments.
Projects are used to track funds and other initiatives; anything for which a giver might make a restricted Gift should be a Project.
Together, these fields allow for tracking of where each Gift comes from (Segments), or how a Gift was raised, and where each Gift's funds are going (Projects).