The layout code is all through pretty basic HTML and CSS (the building blocks of every website on the web... pretty much) but worpress creates the pages through..
From memory a wordpress page is split into four parts -
The header which has all the info in the "head" tags, then usually the top part if the page including the header, navigation and other basic elements. The file for this is a. file (which you can edit, most of it is HTML, just open in a text editor) usually named header. and located inside the folder for the theme that you're using. If you're changing the banner/header this is probably where you want to look so you can find out what the name of it is.
The second part is the "loop" this is a. file that defines how the content is displayed on the screen and the names of the parts into which it is divided, how many posts to show, etc. This can be pretty in depth. so make sure you save a copy before you change it just in case anything breaks :-).
The third part is the footer which contains all the information from the bottom of the content down. Some people have a site map here, designers credits, certifications, that sort of thing. most of this is just basic html as well with a small amount of..
The Final part and MOST IMPORTANT for layout is the css file. The file is also located in the theme directory and this is what defines where everything goes on the page, what the colors are, how they relate to each other, what sits on top of other stuff, all that juicy stuff. It is linked to each element in the html by names, classes and ids. The html file knows where to look for the layout information through a "link" tag in the header (the information part of the page before the content actually starts).
Now - to find out which part of the html file goes with which part in the css file is easy - look at the html surrounding the content you want to change and you will see a property labeled "id" "class" or "name" but usually "id" or "class". it will read something like:
Code:<div id="example"> ... </div>
Code:<table class="example">... </table>
Once you've found that go to your css file and bring up the search function. If the element had an "id" then search for (copying from the above example) "#example". If it has a class then search for ".example". When you find it you'll see a list of properties associated with it. Most of the ones you'll want to change have pretty simple names (like height, width, background-color, border, etc.) so they're easy to find. Some of the ones with crazy names I would leave alone!
Here are some other resources:
Wordpress codex/help and info siteW3 Schools - Great tutorials on most web languagesIf you ever get stuck then google is usually the best place to start. Both those sites are well indexed so if they have the info google usually finds it and if they don't then google finds something else.
FYI You now know more about coding than about half the people that claim to be web designers... :-)