What is the difference between a rail and a picket when ordering lumber?

Georgia Dees: rail is lumber that spans between the post,you dont need pickets for split rail fence

Emerita Sciandra: It is a split rail fence with pickets. Keeping it simple, it has an upper rail, pickets and lower rail. Some of the rails are the same size as the pickets so I just wanted to know if I order 1"x4" rails and 1"x4" pickets OR two 1"x4" pickets.