The short answer
E12 is the common U.S. candelabra base; E14 is a larger small Edison screw used in many imported fixtures.
SpecificationE12E14
Nominal diameter12 mm14 mm
Typical marketU.S. decorative fixturesEurope and imported lamps
Physical fitSmallerLarger
Interchangeable?NoNo
Before you substitute one for the other
Adapters add length and can change the fixture's electrical and thermal behavior.