Span is the distance a beam, joist or rafter bridges between its supports. Clear span is the open, unsupported distance measured between the inside faces of those supports — the part of the member actually hanging in the air. A joist resting on two beams that are 12 ft apart face-to-face has a 12 ft clear span, even though the joist itself is longer because its ends sit on the beams.
Builders use several related “spans,” and mixing them up is a common estimating mistake. Building span is the overall width of a building measured from outside of wall to outside of wall; roof trusses are ordered by this number. Rafter run is the horizontal distance from the outside of the wall to the center of the ridge — half the building span minus half the ridge board. Rafter length is the sloped length along the rafter, which is run × the roof pitch slope factor.
The IRC span tables for joists and rafters list the maximum allowable clear span, and for rafters it is the horizontal projection, not the sloped length. To use a table, look up your species and grade, spacing (16” OC or 24” OC), and design live load, then pick the smallest member whose listed span meets or exceeds yours. A span that just barely passes will meet code but may feel bouncy; going up one size is cheap insurance.