I've been working to something related to address validation for some time I figured I should share this tid bit about the Address1 and Address2 in forms (especially online forms)

When asked for personal information it is not uncommon to be prompted for the following:

Name, Address1, Address2, City, State, ZIP

What really is the difference between Address1 and Address2 and is it significant?

NO, Address2 is not asking for a backup of whatever you put in Address1. It not like a "confirm" address field. Nor it is simply a "continuation" of Address1

It depends on the country but generally (e.g. US)

* Address1 is expected to have Street Number, Street Name, or maybe PO Box.

* Address2 is for Apartment, Floor, Suite, Bldg # or more specific information within Address1

And also, if there is no company field Address2 will be a better place to write it on than Address.

Safe to say that Address1 should be general (but enough to pinpoint a geographical location - and obviously not include City, State, ZIP, Country) while other extra information should be in Address2.

Address2 fields in forms are generally (and for usability) shorter and in my opinion should have at least include hints as to what goes to it.

Also, for US ZIP code lookup you might find the following link from USPS helpful : USPS ZIP Code Lookup

Hope this info helps.