Botanically speaking, an annual is a plant that goes from seed to seed in one season and then dies. True annuals cannot be taken inside for the winter.

In the gardening world, the term "annual" is used to designate any plant that is grown for one season and then dies at frost in the temperate regions of the world. In this case, the term also includes plants such as pelargoniums (geraniums), coleus and impatiens which are, botanically, perennials in that they normally live more than two years in nature. They are used as annuals since they are not winter hardy in the colder climates.

To make things even more confusing, we have the term "bedding plants" which will include tropical plants, vegetable seedlings and "annuals" discussed above.  Stranger, to me anyway, is the term "half-hardy" perennials which you see in British books. It seems to me that tender perennials is a better choice.

Below is an index of plants commonly grown as annuals (of course some are perennials in warm climates) in the home landscape: