Quoted from Transport Canada "Class E airspace is designated where an operational need exists for controlled airspace but does not meet the requirements for Class A, B, C, or D."
Class A airspace requires you to be at a certain altitude to actually be in Class A airspace.
Class B is the same, Class C requires ATC clearance for both IFR and VFR.
Class D just requires VFR to have dialogue (two way communcation) with ATC.
What requirements wouldn't be met in a particular airspace that would need to designate it as Class E instead of the other classes?
EDIT: After some further research, the easiest way I've thought about Class E is an area that requires some control strictly for IFR traffic, but to VFR aircraft it's pretty much uncontrolled airspace.