DEAR HARRY: I had never used my computer to buy merchandise on the Internet, so I'm not sure I did the right thing. I went to a site that I know is legitimate and ordered about $150 worth of Christmas gifts. I played it safe and gave them my credit-card number over the phone rather than send it on the computer. However, the lady I spoke with asked for the three-digit code that appears on the back of the card. I saw no reason not to give it to her, but I am curious as to just what it's for.