A rigid body is said to in equilibrium if the resultant of all forces and all their moments taken about any and all points are zero.
Free body diagram (FBD): depiction of an object with ALL the external forces acting on it.
Reactions coming from the supports: usually a body is constrained against motions using supports. It is essentially to correctly estimate the number and type of reactions that a support can provide.
A slide-share presentation about equilibrium can help you to understand the key concepts of this chapter: