Annual plants are defined as herbal plants as opposed top wooden plants with a life of just a few months and which, once flowered and dried, can be pulled and thrown away.
In warmer climates, as for instance around the Mediterranean, where there is little or no frost, many of this species can survive the winter and flower the following year. Examples of these are: Amaranth, Petunia, Forget-me-not, although you should note that the flowers will not be of as good a quality as in the first year.
A progressive deterioration takes place.
A short list of annual plants is as follows:
Agerato, Amaranth, Antirrino, Calendula, Cresta de gallo, Cosmos, Clarkia, yellow Alhelí (Cheiranthus cherii), Coronado (Callistephus chinensis), Clarkia, Cineraria hybrida, Digital (Digitalis purpurea), Sunflower (Helianthus annus), Alegría de la casa (Impatiens walleriana), Lobelia, Petunia, Banderilla (Salvia splendens), Tagetes, Zinia, etc.
It does not mean that they flower every two years, it simply meant that their planting and flowering is in two different years. For example if you plant them in the summer or autumn of on year, they will flower in the spring of the following year, after winter as they just need more time before they flower. There are not many biennial species about 8 or 10. Again, depending on the climate, they could become a permanent feature in frostless zones.
Both annual or biennial plants are very colourful, in all colours imaginable and they do flower during all seasons of the year, although more in summer of course. They may need to be planted every year, but they are very inexpensive in seed form and they do yield high rewards and satisfaction.
The other advantage is that the appearance of a garden can be changed every year with very little effort and cost, thus enabling to experiment each year until a satisfactory colour scheme is found. Finally they are very useful to fill empty spaces that were not planned for and that would spoil the look of the garden if left empty. The climate we are in is ideal, so make good use of it.