The masterpage is what defines the design of your site. It applies for all pages within that site, except for the application pages which are shared by all SharePoint sites in the farm and use a different master page. You can recognize the application pages by looking at the url... it contains /_layouts/ (a virtual directory shared by all SharePoint sites).
When you create a new page in a MOSS site that has the publishing feature enabled, you are asked to specify a page layout. For example: 'Article page with image on right'. Each page layout is bound to one content type, in this example the content type 'Article'. Each content type can have multiple page layouts bound to it. You can have an article with the image on the right, with the image on the left or even without an image.
The content type is what defines the metadata for the page. An article for example, has a title, description, page image, article date,...
The page layouts are stored in the Master Page Gallery. You can create page layouts for existing content types, or you can create a new content type.
Basically, the master page defines how your site looks like, the page layouts define how your pages look like. One way (the easiest way) to create page layouts is to use SharePoint Designer which you can download for free from the Microsoft download pages.
You can find a lot of information about creating page layouts on the web. This may be a good starting point: http://office.microsoft.com/en-us/sharepoint-designer-help/create-a-publishing-page-layout-HA010174128.aspx