If you are using Workbench and Mechanical, the default "Program Controlled" element order is quadratic for almost all structural models. If you want to change that, click on Mesh and in the Details window is Element Order and you can set that to Linear or Quadratic.
Quadratic means there are mid-side nodes. Linear means there are not.
Quadratic elements are generally preferred because they capture the curvature in the solution along the element edges and across the element faces.
Sometimes Linear elements are required. Some material models require Linear elements, such as the Gasket material model. Some numerical methods such as the nonlinear adaptive remeshing requires linear elements (at least in 18.2).
There are more examples of when to use Linear elements in this portion of the ANSYS Help system.