Thin Beam vs. Thick Beam

Standard beam theory (Euler-Bernoulli bending theory) assumes no deformation by shear. This can be safely applied to thin beams that are long and slender. For short and deep beams however, displacement due to shear becomes hugely important which is included in the formulation of thick beam element. Below is an exaggerated example of shear deformation:

Displacement due to shear that standard beam theory does not take into account

     This effect is the key that distinguish between the Euler-Bernoulli and Timoshenko (thick beam theory) bending theories. See below for a direct comparison between thin and thick beams.

Table illustrating the differences between thin (Euler-Bernoulli) beams and thick (Timoshenk) beams. Diagram in "look" section is adapted from Wikipedia.

Also read
Types of elements in the element library