Hi Karen,
It is the (X)HTML/CSS coding behind the scenes that does this. You might see "template-driven" sites that are basically a box centered in the middle of the page at a specific width (most generally using tables [which I don't recommend]). Using (X)HTML and CSS you can design sites that let you think outside the box, so to speak.
I use BlueHost for all my domains and hosting, so I'm not limited to templates and use HTML/CSS for my sites (well, except for my new VA site which I am using WordPress - but (X)HTML/CSS are still behind it, not tables

).
Gosh...did I answer your question?