The slant: a slant is a 45 degree cut straight across the field with your hands up the entire time because the abll will be common hard and fast.
The Square-In: a square in is when you run a determined amoun of yards down the field straight(im going to use 7 as an example). So you would run seven yards down the file then cut perpendicularly across the field and catch the ball.
The post: For a post you would run a determined amount of yards down the field straight(again i will use 7). Once you ran seven yards you will the run towards the post usualy about a 45 degree angle from the straight 7 yards you just ran, then catch the ball.
The Fly: While running a fly you would run straight down the field looking back at about the 10 yard mark to adjust yourself to where the ball is goin and catch it.
Those four routes are use to gain all kinds of yardage. The slant and square in will get you short-medium yardage and