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Site column and field type
They are different concept. Site column is higher level concept, and field type are lower level. Site column reference field type. Site column examples are City, Company, FirstName. It can be reused when create new content type and list. Field type examples are Text, Choice, Notes. But WSS3 allow user to create a customized field types. The motivation for doing this is to gain a greater level of control over initialization, rendering, and data validation that goes on behind a column.
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A backup issue linked with site definition
In WSS, every site is provisioned from a specific site definition. This is true for all top-level sites as well as child sites nested within a site collection. Once a site is provisioned from a particular site defintion, it picks up a dependency on that site definition that remains in effect for the lifetime of the site. A site's dependency on its underlying site defintion can never be removed or changed, and the site definition must be installed and remain functional in the farm for the site to continue working properly.
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Page Layout
Master pages enable developers and designers to define the overall look and feel of the Publishing site with just a single file, along with some additional branding files such as CSS or images. Just as in ASP.NET 2.0 sites, SharePoint sites also leverage content pages that fill in the content placeholders defined within a master page. Publishing sites take this a bit further by introducing a type of content page called a page layou. Page layouts, when combined with the master page, define the rendering and layout of a page. When the page layout is requested, SharePoint fetches the master page referenced within the SPWeb.CustomMasterUrl property and merges the two together. Developers and designers use page layouts to host editable regions of a page, implemented with Web Parts and field controls.
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A safe master page
The master page use contentplaceholder as reserve space for the client to fill in it with customized content. If you develop a master page with a placeholder, it is optional for the content page to fill in the placeholder. If the client fill in a placeholder, which is not defined in master page, this will throw an exception. So to be safe, the master page, should have all the placeholder that all client pages use. MSDN has an article about How to create a minimal master page. But is is not necessary, if you develop a master page which will never be applied to the content page out of the box, and it is only used by your customized content page. In this case , you have full control of what place holder be put in the master page.
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synchronize master page for application page and site page
The master page of application page is under "_layouts" folder, an example is like MasterPageFile="~/_layouts/application.master". Here is trick to demo how to switch it to the site's master page.