Address Book.app on your Mac offers two options for getting information out of your address book: "Export vCard..." or "Address Book Archive..." but you may not have ever known which you should use and which you should avoid as if it were a swarm of bees.
Here's a simple mnemonic: "vCards good, Address Book Archives VERY VERY BAD." Sorry for yelling, but it's true. Address Book Archives can only do one thing: replace your entire address book.
Why is that bad? Let's assume you backup every Monday, and then some Friday you realize that you accidentally deleted some contacts. If your only backup is an Address Book Archive, you have to hope that you won't lose any important information that you've added or changed since Monday.
If you had an export of your vCards, you could merge it with your existing information:
The only real "trick" to exporting vCards is that you have to be sure to select all contacts before choosing
File » Export » Export vCard...
Also, if you want to see what Address Book is going to do, click the "Review Duplicates..." button (above) and then click "Keep Both" to see the Old vs New information:
vCards are also a good choice because they can be used by a variety of different programs, whereas Address Book Archives are only useful for Mac OS X's Address Book app