Annuals are flowering plants that live their life in one season, or annually. Some can reseed themselves and extend into another season but as a rule annuals will last one season. Annuals are usually the showy and most beautiful ones as well.
When shopping for plants for your garden, most nurseries will group annuals and perennials in their own areas. The tags should state whether the plant is an annual or a perennial. If not, you can always look it up.