Forgive me if I'm wrong, but I think 'head voice' is when you sing higher up in your vocal range and you can sometimes feel it buzzing a little.
Your chest voice is when your sing lower down in your vocal range - it's usually a lot stronger and more powerful, and it's generally easiest to sing in your chest voice than your head.
Most people who haven't had much vocal practise find that their head voice is a lot quieter and weaker than their chest voice too. The way to help this is by practising singing high, and to practise singing in your chest voice and then going into your head.
If you find a note on the keyboard that you can sing really easily and strongly and gradually work your way up, you should hear and/or feel what I'm on about!!!
Also, you asked about expanding your vocal range. One word of advice is DON'T RUSH THIS!! If you force yourself to try and sing much higher or lower than you are used to, this can damage your vocal chords!
Make sure you do vocal warm-ups every day (or if not, at least before you sing).
Start off with some breathing exercises...take lots of deep breaths and concentrate on breathing down to your stomach. It sounds strange, but honest it helps.
Then hum some scales - but don't strain your voice - just relax. Start low and work your way up.
Then sing some scales and sing 'lah' or make up your own sounds like 'mia' or 'yah' or something!! And if possible try to smile because this helps your resonators to work properly! Again start low and gradually get higher.
Now you are ready to sing!! And if you practise this each day or as often as possible, you will find that the higher and lower notes will get easier to sing, and then you can slowly introduce more notes. The key to this is time....don't rush it! It will take a while, but you'll get there one day! If it doesn't seem to be working, keep at it, because I can now sing 3 notes higher than I used to be able to do....it took me about 9 months or so!!
Hope that helps,
Love Laura
ps - another tip -when you sing higher think low, and when you sing low think high! - because this will help your voice relax and you'll reach the note easier and it'll be so much stronger!
A head voice (this is pretty much the same as the other replies) is what you sing in after you hit your high break...the break is the point when it is physically necessary to switch from singing "normal" (chest voice) to your head voice. My break is around an A (above the stave), and it's terrible, but after lots of practice people can smooth theirs out.
One thing I want to add - The area between your head voice and chest voice is usually the weakest notes any singer has. WOrk on strengthening those with songs that cover a wide range.
Breathing exercises and scales help a lot too.
Once you figure out what your head voice and chest voice cover, try expanding your range with going a half step lower. Don't stress it though!
Good luck!