Definition: A true annual is a plant that completes its life cycle in one year. This means it goes from seed to seed and then dies off, during the course of one growing season. The whole mission of an annual is to produce seed and propagate. That's why deadheading or removing spent flowers before the seed matures, produces more flowers and therefore more potential seed.
Some tender perennials, like the popluar zonal geraniums (Pelargonium) are grown as annuals in colder climates.
For a perennial to be worth growing as an annual, it must flower profusely in its first year of growth. Pansies, lantana and alyssum are all actually tender perennials.
There are also plants considered to be hardy annuals. This just means that they are able to withstand a little frost without being killed off and will continue to bloom and set seed into the next year, but they will eventually expire. Bachelor Buttons and Salvia Victoria are examples.
Annuals can be further divided into cool season and warm season. Pansies will fade as the summer heats up. Zinnias won't even get moving until the nights stay warm.
Examples: Annual flowers give you the opportunity to have a totally different garden every year.