An annual plant is one that has its entire life cycle completed in one growth season, according to the University of Illinois Extension. Annuals with seeds planted in the spring typically die out by the fall frosts.
Annuals are categorized as tender, hardy or half-hardy, depending on their fortitude. Tender annuals need warm air and soil, while hardy annuals can survive in cold air and soil. Half-hardy annuals can stand cool weather and soil conditions. Annuals are used in many gardens to help supplement arrangements. Some annuals may act like perennials, plants that come back year after year, by dropping seeds that take root and flourish during the next growing season.