Tubing

There are differences between pipes and tubes:

Both tubes and pipes are measured by: inside dimension ID, outside dimension OD, wall thickness WT
A tube is measured by its OD. A pipe is measured by its ID.
Tubes are used to construct something and pipes are used to transport something.
Tubes come in different shapes: cylindrical, rectangle, round, square. Pipes are always cylindrical or round.
Tube thickness size increases in a fixed increment. Pipe thickness size is based on schedule, no fixed increment.
Tubes normally come in smaller sizes, up to 10" is rare. Pipe can come from 1/2" to several feet.
Tubes is less labor intensive. Pipe is more labor intensive.

standards

ASTM A500 / A500M - Standard Specification for Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes
ASTM A513 / A513M - Standard Specification for Electric-Resistance-Welded Carbon and Alloy Steel Mechanical Tubing
ASTM A847 / A847M - Standard Specification for Cold-Formed Welded and Seamless High-Strength, Low-Alloy Structural Tubing with Improved Atmospheric Corrosion Resistance
ASTM A1065 / A1065M - Standard Specification for Cold-Formed Electric-Fusion (Arc) Welded High-Strength Low–Alloy Structural Tubing in Shapes, with 50 ksi [345 MPa] Minimum Yield Point
ASTM E213 - Standard Practice for Ultrasonic Testing of Metal Pipe and Tubing
ASTM E273 - Standard Practice for Ultrasonic Testing of the Weld Zone of Welded Pipe and Tubing