What is the difference between a welded tube and seamless tube?

As the question suggests, a welded tube will contain at least a longitudinal weld running the length of the tube where the edges of a formed flat sheet are joined together. For tubes of much thicker section, where they are fabricated from plates, there may also be radial (belly) welds to create longer tubes from the shorter plates.

A seamless tube is extruded from a forged bar and so will not have any welds or joins. As welds, or the area adjacent to the weld, can be a weak point in terms of mechanical or physical properties, then seamless tube is seen as a higher performance product. Due to the nature of production, concentricity, dimensional tolerances and finish are typically better too.