How much longer will I have my braces for?

I had my braces put on about 10 months ago and my top teeth are beautifully straight and my bottoms will only need a few more weeks to become perfectly straight. At my next appointment the orthodontist is putting "buttons" (what are they?) on my teeth. What does this mean and how much longer does this indicate I'll have my braces? Oh and he has stopped tightening my top brace when I go the othdontist. THANKS :P x