Bookworm, et al...
Runway heading is exactly what it says - the compass direction or QDR when on the centreline. Flying runway heading after take-off means flying that heading and making no allowance for drift. If a howling gale blows you 20 degrees off the runway TRACK, so be it, just so long as everyone else is doing the same! SIDS in Canada tend to be runway heading to a given altitude, and can be quite entertaining at times.
Straight ahead means following runway track after take-off, compensating for drift.