There are 10 standard Medigap policies to choose from, named after various letters of the alphabet. Medigap A is the most basic "core" policy, offering 100% coverage for Medicare Part A coinsurance and hospital costs, Medicare Part B coinsurance or copayments, blood, and Part A hospice care coinsurance or copayment.
The other nine policies cover those benefits at various levels and may offer additional benefits. Medigap D, for example, also provides 100% coverage for skilled nursing facility coinsurance and Part A deductibles, and 80% of foreign travel emergency health care costs. Medigap M, by comparison, offers the same coverage as Medigap D, except it only covers 50% of the cost of Part A deductibles.
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.