Cover 7x is a trap coverage which is used to double cover a wide receiver (the "X" receiver, hence the name). There are many variations of how to play it: one is a fist technique; a thumbs technique; a slice technique; a trap technique; a swipe technique and a push technique. I could explain all of them in my new thread! But all these calls are relevant based on the offenses formation and how exactly you want to double team the receiver.
This coverage is used a lot by Rex Ryan and actually is the main reason Darelle Revis is so successful. A lot of people think he just locks guys down (which he certainly does, don't want to take anything away from the guy), but if you really pay attention, he gets help more often than you might think.
Darrelle Revis battles Dez Bryant 2011 Opening Night - YouTube ...Check out 9:20 of this video for an example of this coverage. This was opening night against the Cowboys, Cover 7x with a Trap call. The safety flies over the top and Revis is looking for a quick slant or out route. Once Romo rolls, Revis just trails the play from behind using inside-out leverage and the ball falls in his lap.
Cover 8 is a pattern-read concept that is used by every team in the game, primarily out of an Under front. To keep it brief, it's the best way to get safety run support but still have good secondary coverage. The safety reads the #2 receiver and depending on his action, his job will vary. If he runs to the flat or goes underneath, the Safety will rob #1. If #2 goes vertical, the Safety plays him man-to-man. If he blocks down, the Safety is the force defender; if he blocks out, the Safety fills the alley. The corners play deep man-to-man on #1 unless they go underneath the formation within the first three yards. The linebackers play it like Cover 4.
Any team that uses a two-deep safety look will use Cover 8 from an Under front against a team that runs with two backs in the backfield. It has become the go-to method, especially with an NFL safety like Troy Polamalu or in college a guy like Mark Barron who can play well in run support.