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.
Seamless tubing is manufactured from a solid billet which has been extruded into a seamless pipe and is sometimes referred to as a hollow or mother tube. The pipe is reduced to smaller sizes using pilgering and/or cold drawing. Pilgering is also referred to as cold rolling and uses two rollers with grooves to control the outside diameter reduction, with a mandrel to control the internal diameter reduction. Pilgering is extremely fast and reduces cross sections by up to 90% but carries limitations on smaller diameter tubing and is best suited for high volume manufacturing because of the difficult change-over times.
Alternatively, welded tubing is manufactured using flat strip which is roll formed into a circular cross section. It is then longitudinally welded using TIG welding or Laser Welding processes. After the welding is complete, the weld seam, or bead can be left ‘as welded’ or refined by cold working the bead which uses rolling and forging methods. 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. Welded tubing can also be drawn after welding (much like seamless tubing), that results in an even finer weld seam, tighter tolerances and a superior surface finish.