Web Design what have i done?!

ok, first off, i'm a complete noob, so bare with me. haven't read enough yet, i know. but i don't know where to look, not knowing the term for what i've done...
so i've been playing around a html template (charity trust), it looks ok as an initial first off - http://img35.imageshack.us/img35/7611/2010040519h3646.png
but when the page loads in a browser the whole top section is off centre. the red lines are all still in line (don't even know what they're called yet), and i've only been playing with the logo - http://img169.imageshack.us/img169/8353/2010040519h3710.png

been using gimp and kompozer.

anyone know off hand what i've done?
thanks
reg
(been going through tutorial vids, but man, there's a stack! frustrating when you know you want to do a simple adjustment, but don't know the right term/effect to google - i'll get there tho)