A tea bag is a small bag that holds the leaves needed to brew a single cup of tea. The tea is brewed still inside the bag, making it easier to dispose of without a tea strainer. The tea bag allows for a single cup to be made, instead of a whole teapot's worth.
The tea bag was accidentaly invented by American tea importer Thomas Sullivan in 1908. He had sent samples of his tea out to customers packaged in silk bags. His customers put the entire bags into the pot, thinking that was what Sullivan had intended.
Tea bags were commericaly produced in America by the 1920s. The silk was replaced by gauze and later paper. Tea bags took off in the UK by the 1960s - today in the UK 85% of tea consumed is brewed using a tea bag.
A well produced tea bag, with enough space for the tea to infuse properly, is a convienant alternative to loose leaves. However, tea bags are often let down by the poor quality of the tea - small, dusty leaves from many different sources which tend to release tannin more quickly, making the tea taste harsh.
Many people think that loose leaves brew a superior cup of tea, and believe that the ritual of leaves is part of the experience of your tea. Many blends of tea are not available in tea bags, and with loose leaves you are free to experiment with your own creations.