The difference between seamless and ERW pipes

On the surface of seamless pipe, there are no welding or joints and seamless pipes are made from solid round billets. The seamless pipe is finished to dimensional and wall thickness specifications in sizes from 1/8 inch to 24 inch. Applicable in for High-pressure applications such as Hydrocarbon Industries & Refineries, Oil & Gas Exploration & Drilling, Oil & Gas Transportation.

Electric Resistance Welding (ERW) pipe is manufactured by rolling metal and then welding it longitudinally across its length. 

Seamless pipe is manufactured by extruding the metal to the desired length; therefore ERW pipe have a welded joint in its cross-section, while seamless pipe does not have any joint in its cross-section through-out its length.

If the materila standard is ASTM A53, Type S means seamless.