WEF - With Effect From
"With Effect From" is used to represent the starting date of something. For example : If we want to say 'due to financial distress, the company would not be able to pay bonus to their workers from next financial year 2015'. So we could write :-
"due to financial distress, the company would not be able to pay bonus to their workers, w.e.f. 01/04/2015."
WEF - World Economic Forum
The World Economic Forum is a non-profit independent international organization Headquartered in Cologny, Geneva, Switzerland. It was originally named as "European Management Forum" which was founded in 1971, later on, in 1987 the name has been changed to World Economic Forum.