Stress is force per unit area. If σ is the stress, F the force, and A the cross-sectional area, then σ = F/A. Tensile stress and compressive stress are axial forces applied to a body that tend either to extend it or compress it linearly. Shearing stress is a tangential force per unit area that tends to shear a body. See also Hooke's law.
In a fluid, no shearing stress is possible because the fluid slips sideways, so all fluid stresses are pressures (positive or negative).