Why did the remaining money go to another address? I'll try to be short.
I gifted 1 BTC to a friend several months ago. She is a completely non-technical person. She tried to send a small fraction of that 1 BTC to her brother, and either she seriously messed something up, or fell a victim to some attack.. I never had a reason to use the Blockchain Explorer, but now that I want to try and figure out what happened to her 1 BTC, this is what I see when looking up her Wallet's address in there:

1) A transaction where I send 1 BTC to her:

{transaction id xxxx}              ===&gt;    16WE..&lt;her address&gt; (Spent)   1 BTC
1HHj..&lt;my address&gt; (N BTC Output)          1HHj.. &lt;my address&gt;(Unspent) N-1 BTC


That's what I've expected to see - I've started with N BTC, spent one, it's now in her wallet, and I got N-1 left.

And here's the 2nd transaction (of the only 2 associated with her wallet):

2)

{transaction id zzzz}               ===&gt;    14vg..&lt;her bro's address&gt; (Unspent ???!)  0.0001 BTC
16WE..&lt;her address&gt; (1 BTC Output)          15Hn..&lt;WTF? who's that?!&gt; (Spent) 0.9999 BTC


Well, F&amp;#@. Whatever they did (I should've looked over her shoulder...), the dude got the fraction of the 1BTC which was intended for him - but why the rest ended up in some wallet other than the originator's (as it happened in (1) above).

I don't she'd unknowingly did the "advanced send" stuff, which does allow multiple target wallets.. but anyway, she's not technical, but she ain't dumb to place some random wallet into the 'To:' box.. and being very new to the whole explorer business, being a little confused, but I want to know what happened. Why her bro's wallet has "Unspent" next to it, unlike in the first transaction, is just one of the questions. Where could the second wallet come from is another, obviously.

Thanks!
What you are observing is a change address.

The wallet created a new address for itself to send the change of spending the 1 BTC to. This is so that it is non-obvious to the rest of the world which is which. The 0.9999 BTC is still hers to spend, just assigned to a different address.

And it is working: if you didn't know she was sending to her brother or his address, you would not be able to infer anymore how many BTC she holds.


  I never had a reason to use the Blockchain Explorer


You generally never have to. The problem is that explorers do not know your wallet. They present a lower-level view of what is going on in the system than your wallet does, which can be confusing.

Read more: Why does Bitcoin send the &quot;change&quot; to a different address?