A weave and a sew-in are both hair extensions, but the difference lies in the way the hair extensions are attached to the natural hair. A weave is a general term that describes any method of attaching hair extensions to natural hair, while a sew-in specifically refers to a method in which the hair extensions are sewn into the hair using a needle and thread. The natural hair is braided in a sew-in, and the hair extensions are sewn onto the braids. With a weave, the hair extensions can be attached in various ways, such as through glue or clips.