Picture sizes

I have a 4:3 tv. When i play dvds only "half" the screen is filled up. At the bottom and top i get two thick black bars with no picture. Is this normal when having a 4:3 tv? any settings can be changed to fill up the whole screen and at the same time maintaining a clear picture?
On the dvd player manual i read something about Widescreen, panscan etc. Can someone explain the differences please?
ALso whats the difference between pal and ntsc?
On what criteria should choose between the two?