How each is made
Seamless pipe is formed by piercing and drawing a solid billet into a hollow tube with no weld seam anywhere; welded pipe (commonly ERW — electric resistance welded) is formed by rolling flat steel into a tube shape and welding the seam closed along its length.
Where the difference actually matters
For architectural railings and general fabrication, welded pipe is standard, cost-effective and entirely adequate — see our pipes & tubes guide. Seamless pipe's advantage (no seam as a potential weak point) matters most in high-pressure industrial applications, not typical architectural use.
Cost difference
Seamless pipe costs meaningfully more to manufacture and is priced accordingly — specifying seamless for a standard railing project is usually unnecessary expense for a benefit that doesn't apply to that use case.
How to tell which you're getting
Ask your supplier directly and check the specification on your quotation or mill test certificate (see our MTC guide) — this should be stated clearly, not something you have to guess at.