There are many different types of sew-ins, but the most common are partial sew-ins, full sew-ins, and invisible part sew-ins.
A partial sew-in involves sewing in only a portion of the weave, leaving some of your natural hair out. This is a good option if you want to add body and volume to your hair without completely covering it.
A full sew-in involves sewing in all of the weave, so that none of your natural hair is left out. This is a good option if you want to add length and/or thickness to your hair, or if you want to protect your natural hair from heat damage.
An invisible part sew-in involves sewing in the weave so that it covers your entire head, except for a small section in the front where your natural hair part will show through. This is a good option if you want to add length and/or thickness to your hair while still being able to see your natural hairline.