Seamless pipes are derived from solid steel that is in sheet or bar form and is formed into a solid round shape known as “billets” which are then heated and cast over a form such as a piercing rod to create a hollow tube or shell. From: Reliability and Maintainability of In-Service Pipelines
Welded pipe is also called ERW (Electric Resistance Welded) pipe. Most common specification for welded carbon steel pipe is A53. Seamless pipe is made by a process that requires no welding. Most common specification for seamless carbon steel pipe is A106B.