What is the difference between the canon 420EZ and canon 420EX?

The EX series of speedlites handles e-ttl metering whereas the ez is the older model and does not. The ex also supports the seven sensor AF points of the EOS Rebel Ti, Elan 7, 10D, Digital Rebel, and Digital Rebel XT. If you are using a canon camera which is newer than five years old or so, you want the EX. What is E-TTL, you ask? Essentially it's a way of controlling flash duration so that your photos are properly exposed, even if the flash head is pointed away from the subject. You definitely want it.