What is wide flange beam in construction?

No. The 12 is a nominal depth; a W12x26 is actually about 12.2 inches deep, and depths vary within a series because heavier shapes in the same nominal group are rolled deeper. Always pull the actual depth from the AISC Steel Construction Manual or the shop drawings when checking clearances, deck bearing, or MEP routing, never from the designation alone.

Structural steel is conventionally bought and priced per short ton of 2,000 pounds, with fabrication and erection quoted per ton as well, so the takeoff converts every member length to pounds using the lb/ft in its designation. Small packages and miscellaneous steel sometimes price per pound or per piece, but the per ton convention dominates US commercial work.

In the field, people call both an I beam, but they are different rolled shapes. The W shape has wide flanges with nearly parallel faces and carries most modern beam and column work, while the American Standard S shape has narrower flanges with sloped inner faces and shows up mainly in older buildings, crane rails, and monorails. Schedules call out the exact designation, so take off whatever letter the schedule shows.