Pitchers have been developed in three distinct and unrelated families.

They are filled with water and often also enzymes (depending on the species) to digest the insects faster.
Many people think that the lids of some species close after capturing an insect. However an insect that falls into a pitcher rarely is able to escape regardles of the lid. So closing the lid only costs a lot of enery and prevents other prey from falling in. So none of the pitchers close.
Especially within the genus Nepenthes studies have learned that these plants have found even more ingenous ways to gather nutrients. This is way to much info to place on this photosite. If you are interested in reading more about this I can strongly reccoment the books Pitchers plants of the old world, Pitcher plants of the new world by Stewart McPherson, Any of the Nepenthes books whritten by Charles Clarke and the book Pitcher plants of Borneo by Phillips, Lamb & Lee.