Spinach is considered to be an annual plant because of the fact that they flower and produce seeds then die. Annual plants do not return to the garden on their own. Perennial plants on the other hand bloom, die, and then go dormant until they return to the area where they are planting the next season.
You will find that there are a number of benefits to growing annual plants, including the fact that they come in a wide variety of colors and sizes, so you will have quite a bit to choose from. One of the drawbacks of these plants though is that you have to put in a new crop of spinach plants each year at the appropriate time.