All good websites have defined structures used in designing the layout. Best way to look at it is as if you are writing a format letter. There are guidelines on how you write a formal letter depending on what type of letter and who the letter is being addressed to, same with web pages. A page layout can be broken down into sections as seen in the diagram below:
Header section is similar to your heading in a letter. It contains the logo of the website and navigation bar Main Menu also any addresses you would like the viewer to reference.
Usually situated in the Header section and is a very important feature of the header. This is usually a horizontal navigational me nu that allows viewer navigation through various sections of the website.
Side bar are also called secondary menus, they are usually vertical navigation bars that allow the viewer navigate to other pages of interest in the site. Believe it or not there actually is a science or a well thought out process to building up navigation hierarchy of a side bars.
This is where most of the content is found like the main body of a formal letter after a salutation to the addressee.
Footer is another important section of a web page. It usually contains information related to contact information for any organizations, or declaimer notifications, or links to relevant external site. Most importantly the footer contains a footer navigational menu, usually is meant to be redundant to the header menu. Its supposed to give the viewer an idiot proof way of navigating through the website.