That being said, understanding Cover-2 (and it's variants) is still extremely important. Cover-2 is one of the building blocks of the modern NFL, and it has a long, storied history.
In the above image, you see the basic Cover-2 zone with its defining characteristic, the deep half safeties. As its name implies, Cover-2 splits the deep part of the field in half, leaving two large zones for the safeties to cover (landmarks are just outside the hashes).
Underneath the safeties, the three linebackers are tasked with covering intermediate zones that equate to roughly 1/5 of the field each. The MIKE's zone is the middle hook. He will "open" his hips to the strong side while protecting the middle of the field from crossers and carrying the #3 receiver vertically. The WILL and SAM, meanwhile, have hook/curl responsibility. They'll protect the seam and break on underneath routes like the curl route.
Finally, the corners have curl/flat responsibilities. Here, they're shown in press coverage, and that's fairly common in Cover-2. The corner will jam the receiver at the line to hopefully disrupt his timing and force him inside, rather than letting him release outside (thus cushioning the "honey hole", the space behind the corner along the sideline). However, by no means do the corners always play press in Cover-2. Often times, it differs based on coaching and the individual match-up.
Cover-2 corners are often force players (or edge setters) against the run. Therefore, a Cover-2 corner can't shy away from contact.
Tampa 2
Tampa 2 is a variation of the Cover-2 that was perfected by Tony Dungy and Monte Kiffin when they coached together in Tampa Bay. Dungy first got the idea to drop the MIKE into a deep zone when he was playing for Bud Carson and the Pittsburgh Steelers. Carson had a number of athletic, instinctive outside linebackers, so he decided to give them more responsibilities while sending his inside linebacker into coverage to help the safeties.
Let's take a look at the Tampa 2 on the chalkboard:
In reality, Tampa 2 is more like Cover 3 than Cover 2, in the sense that the field is broken up into three deep zones and four underneath zones.
There are some important differences between Tampa 2 and Cover-2. Most notably, the MIKE gets to drop "down the pipe" and cover the deep middle of the field. As you can imagine, this requires an incredibly athletic inside linebacker (think Brian Urlacher, who may have been the best Tampa 2 MIKE of all time). Additionally, the safeties' pre-snap alignment is wider in the Tampa-2 (top of the numbers) than in Cover-2, making it easier for them to drop into their deep third zones and protect against the #2 corner route.
The WILL and SAM 'backers still have hook/curl responsibility, but their zones are now bigger than in the standard Cover-2. Finally, the corners will align off the ball more than their Cover-2 counterparts. They're often the "force" players against the run, and their soft alignment gives them time to diagnose the play and space to avoid blockers.
Outside References
1. Defensive Back and Linebacker Responsibilities With Cover 22. Football 101: The Cover-2 Defense
3. Understanding the Cover 2 Zone
4. Defensive Back Techniques: Cover 2 Pattern Read Examples
5. Guide to NFL Defenses, Part 3: The 4-3 Front, Continued
6. Football 101: Breaking Down the Cover 2 Defense
