The very definition of a project includes a set of inter-related activities done by a group of people (according to the PMI®). Activities are linked to each other using relationships in the sequence in which those activities must be performed. There may be one or more paths of related activities, but one subset of them will be critical, meaning that the rest of the schedule is driven by the activities on that path.
There are four ways to relate activities to one another (types of relationships):
Finish to Start
Start to Start
Finish to Finish
Start to Finish
Relationships will be described using three abstract activities, called A, B, and C.
This is the most logical and most often used relationship type. In this case, A must finish before B starts. This is an expression of a relationship between these two activities only. For example, the frame of a building must be put up before any electrical work can begin; or the rig must be assembled before drilling can begin...