Receivers come in all shapes and sizes. Here are each types:
Possession - These players usually aren't the fastest on the team BUT they have great routes and the best hands. These are your sure hands for a first down. They are very smart and know how to use their body to get open. They can really be any height but usually they are preferred to be a bigger body since they aren't that fast. They usually play on the outside. Ex. Michael Thomas.
Slot (Fast) - These players are usually the fastest of the WRs and you want them to get the ball in open space and run for touchdowns. It's a huge plus if they have great hands and great routes but in a lot of cases they don't need to be the best at either because they are so fast and their passes are usually thrown very short. They are typically the shortest of your WRs, sometimes your punt returner as well. You'll always find them in the slot. Ex. Tyreek Hill.
Slot (Crafty) - These WRs are similar to the possession receiver but are shorter guys with most likely the best route running on the team. These WRs are the most intelligent because they don't let their speed or height keep them from getting open. You'll always find them in the slot as well. Ex. Julian Edelman.
Big Man / Red Zone Target - These WRs are some of the taller guys on the team, typically very athletic. Their routes and hands aren't as important as their ability to win 1v1 match ups. You just toss the ball up to them and most of the time they come down with it. You'll always find them outside. Ex. Mike Evans.
WR1 - These WRs can essentially be any of those types above. They are just your best overall wide receiver and best playmaker. They can be any height but are usually a fast but don't need to be the fastest. You'll trust them to make the big play and make the 1v1 catch as well. You'll find them in the slot and outside.
Freak - These guys are the freakiest athletes on the field, very tall, athletic, fast, great route runners, and have great hands. Defenses usually make gameplans to stop them. They can play slot or outside. Ex. Julio Jones.
Now that we know what different types of WRs there are, we can get into what they do.
Every WR must learn the route tree. The route tree is a group of patterns that WRs must know how to run on the field. Every offense uses some combination of these routes in their passing game.
These are the most basic versions of each route