There are 12 standard Medigap policies to choose from, with the eye-popping names of A through L. Medigap A is the most basic "core" policy. As you move through the alphabet, the plans add more coverage. For example, Medigap E will offer something that is not included in Medigap D, but will lack a coverage provided in Medicare F.
There is no difference in plans offered by different insurers; plan details are all set by the government. (Important caveat: If you live in Massachusetts, Minnesota or Wisconsin, check with your state insurance company or a private insurer who operates in your state. Medigap policies in these states offer coverage different than the plans followed by the 47 other states.)
If you and your spouse want Medigap coverage, you'll need to buy separate policies; spouses aren't covered together. The cost will vary depending on where you live, your health and, of course, the specific plan you choose.
To compare Medigap plans, go to the Medicare Web site.
To learn more about how Medigap works (and get a quick tour through the world of Medicare), pull up a chair, settle in, and wade through this 56-page brochure that walks you through all your options. It also explains how a Medigap policy interacts with other insurance, such as Medicare and any employer-provided benefits.