A caul is the membrane that encases a foetus as it grows inside the womb. This foetal sack (or amniotic sac) is usually torn during the labour process and is peeled off and discarded after the newborn baby has been cleaned. Not a very tasteful subject for an entry, especially if you're eating your lunch.
However, when a caul covers the baby's face, it is often preserved by superstitious people. This facial caul becomes an omen of a dubious nature for both the baby and for whoever possesses the caul after it has been preserved. A caul can be preserved by drying it out or smoking it - part of its usage meant that it had to be easily worn/displayed.
In the mythology of Eastern Europe it was believed that if your child was born with a caul, then it would grow up to be a vampire/werewolf. To stop this from happening the caul would be displayed prominently in the household. If a werewolf was believed to be attacking a certain area, then the caul would be tacked to the entrance of the house so that the beastie would know that the family were already cursed and thus leave them alone.
In Western Europe and the Americas, cauls were seen as a positive omen and sailors used to present them to their captain in order to protect the crew and ship from harm. It was considered extremely fortunate to enrol a sailor who was born with a caul onto your ship and such seaman were actively sought/press-ganged. The tradition is well chronicled and appears in Charles Dickens' David Copperfield:
I was born with a caul, which was advertised for sale, in the newspapers, at the low price of 15 guineas. Whether seagoing people were short of money about that time, or were short of faith and preferred cork jackets, I don’t know; all I know is, that there was but one solitary bidding, and that was from an attorney connected with the bill broking business, who offered two pounds in cash, and the balance in sherry, but declined to be guaranteed from drowning on any higher bargain.
O Ye of Little Faith
The Merseyside Maritime Museum in Liverpool is the proud owner of the caul which belonged to Seaman James Fairclough, who was born with it in 1913. Fairclough carried the caul with him on during his naval career and was blessed for doing so - he survived two torpedo attacks.
Cauls are still actively sought by many sailors and occasionally appear on online auction sites. The Maritime Museum should pay heed to one caveat though - the caul should be buried or burnt with its owner in order for them to rest in peace.