18 AWG stands for 18 American Wire Gauge, the standard measurement for wire thickness.
18 WAG is not a standard wire gauge term. It's almost certainly a typo or misprint for 18 AWG.
For speaker wiring, 18 AWG is suitable for:
Short speaker runs (up to about 25–50 ft / 8–15 m, depending on speaker impedance and power).
Typical home speakers and car audio installations with moderate power.