Xylem vessels are a long straight chain made of tough long dead cells known as vessel elements. The cells are arranged end to end and the cell walls which would normally surround the whole cell dissolve where ever they are touching another cell wall. This makes a tube (because there is no blocking cell wall); sometimes the cells are just filled with holes rather than have no blocking cell-wall.
They have a lignified cell wall and a large central cavity. The vessel cells are also devoid of protoplasm. Vessel members are interconnected through perforations in their common walls. The presence of vessel is a characteristic feature of angiosperms. The secondary walls of vessels are filled with lignin, a kind of hard glue.
Xylem vessels work because of the transpiration stream. It is caused by the evaporation of water from the cells in the leaf to the atmosphere. This transpiration causes millions of tiny rings of cartilage to form in the cell wall of the leaf. The resulting surface tension causes pressure in the xylem that pulls the water from the roots and soil. This is called capillary action.