There are two types of dahlias--those that are grown from seed (and thus considered an annual), and those that grow from a tuber (root in the ground, like a potato) and so return year after year. I am assuming that yours is the latter. How to care for them would depend on what climate zone you live in. If winters are mild you may leave your tubers in the ground. However, if your ground freezes deeply and/or becomes very soggy with harsh precipitation, then you will need to dig up your tubers and store them during the winter in a sheltered place, such as an unheated garage or garden shed. The website sunset.com is a good source of information for gardening questions.