Ultratress is one brand of seamless extension/hair. The concept is a double-sided tape that adheres to the hair. Thin slices of your hair can be sandwiched between strips of seamless extension hair wefts or a liquid adhesive can be added to the scalp for hair replacement. If the seamless hair is applied to the scalp then it is more considered a skin weft. You can not use conditioner at the roots. Conditioner as well as alcohol from "dry shampoos" can cause the tape to loosen and slip. (i.e. extension hair falling out.) This procedure is a great "break" from strand by strand hair extensions.
This is a picture of a traditional woven weft of extension hair. There are little to no product restrictions, but it can be harder to blend with one's own natural hair.
The third picture featured in this article shows both of these human hair extensions used to create one look. Notice: The front 2 inch block of extension hair shown here are woven weft extensions using the gluefree quickweave approach. The back 2 inch block of extension hair is seamless hair extensions. Adhesive is built into the extension hair.
In the far distance, you can see the back of the double sided tape from the seamless hair extension.