Pipe is rigid and resistant to bending whereas some tubes such as copper tubes and brass tubes can be flexible. However, in structural projects tubes are rigid.
Pipes are classified by schedule and nominal diameter. For example, a 250mm nominal diameter and schedule 80 pipe.
Tubes are classified by outside diameter and thickness. For example, 10mm copper tube 2 mm thickness.
In pipes, all the fittings can be matched by nominal size and schedule. For example a schedule 40 one inch pipe will have fittings specified by the same name. These pipe fittings would not fit a 1” tube.
Pipe is always round or cylindrical. Tubes may be square, rectangular and cylindrical.
Pipes generally start from ½ inch to very large sizes. However, tubes are generally of smaller diameter only. We use a 10 inch pipe but not 10 inch tube.