What's the difference between a Battle Tag and a Real ID?

That answer is pretty much it yes - the major difference between Battle Tag and Real ID is that giving out your Battle Tag is not exposing the email address that you have associated with your Battle.net account.

When Real ID was first implemented a lot of people complained that in order for people to add them to their friends list, they had to give out their Battle.net username (email address), which was a potential security risk.

Blizzard's answer to this issue was the implementation of Battle Tag, which is a name that you choose with a randomly generated number on it (ie: YourName#1993) which you can then give out to allow people to add you to their Battle.net friend's list without exposing your email address to them.

In addition to this, as pointed out in the article you have linked, people added to your friends list with their Battle Tag will only show their Battle Tag, instead of showing the name they have associated with their Battle.net account.

The reason people ask for your Real ID still is probably out of habit, or due to them being unaware that Battle Tag is the same thing but with better privacy - remember that giving out your Real ID means you're giving out the username to your Battle.net account.

actually i was confused because he already added me as a friend before and now he's asking for my realid. so was wondering if there was any other thing besides that

@corroded if he's already added you with BattleTag and is on your friend's list then there should be no need to add you via Real ID - remember that giving out your Real ID means you're giving out the username to your Battle.net account. – pixel Jun 26 at 0:56

However, if he added your character as a friend, and doesn't know about BattleTags, then he might be asking you for your RealID out of ignorance of how things work 'these days'.

What it comes down to, is give him your BattleTag. If that doesn't work for him, ask for his. :) If that works for you, then, something's up...either he's not using it right, or he's trying to get your email address/username for your acct.

But remember, security by obscurity doesn't last and isn't reliable.