System of Forces

However, most of the time we don't deal with all the above equations as they are for the general situation of equilibrium in 3D, While in 2D, three of the equations will not be needed as they are already equal to zero and remains two forces in x and y direction and the moment around the z direction.

Concurrent forces is when several forces are applied to the same point. It has the meaning in itself. That is the forces are concurrent! So what is the point about it? Yoda says: Nice, it is; no moment, there are, only the equilibrium equations for forces can be used to solve a given problem, and guess what if the problem is in 2D then we'll only have two of the force equations used rather than six of them! 

If the equations of equilibrium are enough to solve a structure, then we say that the system of force is statically determinate and otherwise, it is indeterminate (more than necessary number of supports for equilibrium. They are called redundant supports or redundant members, total number of them known as degree of indeterminacy). for example, a beam with two pinned supports is statically indeterminate. Examples of determinate beam types are simply supported beam, overhanging beam, and cantilever beam. This page has a lot more details on this subject. 

A free body diagram is a representation of what is happening in reality that shows all the forces, moments, and reactions, which is the first step to solve static problems. Most common support for beams are roller, pinned, and fixed. Roller supports can only support vertical forces (rolls into a new equilibrium position as a result of any horizontal force). One equilibrium equation (for vertical direction) is needed for this case. For the pinned support, which supports both horizontal and vertical forces, two equations are needed. If the surface that a body touches is frictionless, the reaction toward the force imposed is normal to the contact surface (the same is true for roller and rocker supports).