Cranberry sauce can be controversial. In fact, often enough the issue cannot be resolved unless a family can agree to have two types of cranberry sauce at Thanksgiving dinner - fresh and canned.
Though often treated as a supporting character in the feast, cranberry sauce is certainly the most versatile dish next to gravy, delicious piled atop nearly any other dish. Cranberries were considered a seasonal fruit until Marcus L. Urann, an attorney, realized he harvested more cranberries than he could sell. So, he made a sauce, canned it, and called Ocean Spray. In 1941, the jellied cranberry log was put on the market.
Canned cranberry sauce is made with berries, high fructose corn syrup, water, and corn syrup. As fresh cranberries were once a rare thing in the U.S., the canned relish gained popularity until it became a Thanksgiving staple. If you're interested in seeing Ocean Spray's instructions for removing the log, intact, from the can, they offer detailed steps here.
A log of cranberry sauce, naturally, was not served at the first Thanksgiving - sugar was scarce in colonial times. There may well have been fresh cranberry relish, however.
Fresh cranberry sauce has been gaining popularity since fresh cranberries have become widely available. The berries in fresh relish are generally not cooked, but simply mixed with sugar and anything from orange zest and nuts to ginger and kumquats.
Because fresh sauce tends to be more tart than the canned, it actually makes a lot of sense to serve both at Thanksgiving. A little of one balances out a little of the other. And any leftovers can easily be incorporated into Morning After Cranberry Muffins or Cranberry Ribbon Cake.
Lastly, a little known fact about cranberry sauce - while listening closely to the end of the song Strawberry Fields Forever, John Lennon can be heard repeating the words "cranberry sauce". In an interview, Lennon said that the words have no special meaning except to contribute to the weirdness of the song.