Understanding how raspberries produce is the key to getting the most from them. While their roots are perennial, their canes are biennial, dying after the second growing year. In order to prune successfully, you have to know which canes bear when, and that is a function of whether are summer bearers or fall bearers.
Summer-bearing raspberries act like true biennials; the first year is for producing leaves, and the second year is for flower and fruit production. After that, the dead canes just stand around interfering with berry picking. Every year, raspberries send up new canes, so the penalty for not pruning — rampant sprawl and painful harvests — mounts.
The elegant way to prune summer-bearing raspberries is to cut all canes at soil level in the summer, after they are finished fruiting, and then prune out all but the strongest four or five new canes in the spring, once they are 8 to 10 inches high.
Fall-bearing raspberries, also (somewhat misleadingly) called ever-bearing, produce fruit twice; at the tops of the canes in late summer or early fall of the first year, and again lower down on the canes during midsummer of the second (and final) year.
Prune fall-bearers in the spring. For maximum yields, completely remove the canes that produced fruit during the previous midsummer (look for old, cracking bark on the two-year old canes). Leave enough of the brand-new canes to produce a late, first-summer crop, while removing as many as necessary to control the size of the plant.
Alternatively, you can simply cut fall-bearing raspberries to the ground each spring. You’ll get only the late crop of berries, but you won’t have to decide which canes are two years old and which ones are only one; you simply whack them all back.