A tea bag is a small bag that holds tea leaves, either the amount needed to brew a single cup of tea, or a larger one, of which one or two are used for a whole teapot. The tea is brewed still inside the bag, making it easier to dispose of without a tea strainer.
The tea bag was accidentally 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 commercially 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 convenient 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.
Traditionally, tea bags have been square or rectangular in shape. More recently circular and pyramidal bags have come on the market, and are now quite common. These are often claimed by the manufacturers to improve the quality of the brew, although it is doubtful whether they make a significant difference.
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.
The concept of premeasured portions to be infused in disposable bags has also been applied to coffee, although this has not achieved such wide market penetration (contrasting sharply with the fate of instant tea vs instant coffee).