Beam supports are generally classified as pinned, fixed or free.
Beams span between supports carrying the external load forces to the external reaction forces.
The type of support influences the distribution of bending moments and shear forces. For simple span beams the supports may be pinned, fixed or free.
A pinned support provides vertical but not rotational restraint.
A fixed support provides vertical and rotational restraint.
A free support provides no restraint which might seem to be a paradox (a free support is often called a free end).
The type of support significantly influences the bending moments and shear forces.
For the same span L and the same loading, say a uniformly distributed load of W, the distribution of bending moments and shear forces is quite different.
In simple construction, beam supports are commonly assumed to be pinned.
Ideal pinned and fixed supports are rarely found in practice, but beams supported on walls or simply connected to other steel beams are regarded as pinned. Where beams are continuous or part of complete frames (portal etc.) the distribution of moments and shear forces is influenced by the behaviour of the complete structure.