WAAAGH! is both the Greenskin word for "crusade" or "warpath" or simply "carnage" and the term for the magical energy that powers their Shamans. Rather than being two different interpretations, the term is used completely interchangably.
A WAAAGH is formed when a number of Greenskins unite for a particular purpose led by the strongest orc of the group or perhaps the orc warlord breeds the WAAAGH and the Greenskins are simply attracted to it. Or maybe the WAAAGH was always there and Mork and Gork simply guide Greenskins to follow a particular one. No one really knows, and the Greenskins do not much care, either. All they know is that the more they are, the bigger the WAAAGH becomes and the bigger the WAAAGH, the more Greenskins will be drawn to join it. As the WAAAGH grows, the primal WAAAGH energies grow, and it is these energies that shamans feed upon and channel into magical effects on the battlefield.