here are some - Asked By Venkat K on 29-May-06 04:08 AM

MasterPages provide two distinct advantages: 
1. They provide a clean separation between an ASP.NET Web page's form and function. The form of the page - the associated HTML - is tucked away in the template. So, if you need to, say, update the HTML template for all ASP.NET Web pages on the entire site, all you have to do is modify the template file and all of the ASP.NET Web pages that utilize that template will be updated automatically. 
2. They allow for customization of the site. You could have multiple templates created and allow users to choose from one of a series of templates. When an ASP.NET Web page loads, then, you could dynamically choose the template the page should use based on the user's preferences. 
Disadvantages:
1. your pages all need to have the control, it's a small bugger but still a bugger ;) and 2. your controls shift in the control tree, at runtime they no longer belong to the Page itself, but to the added control. For some (like me) this might be bugging and even interfere with working code.

You can think of a master page as a template. Inside you have placeholders for the specific content. If you take this site as an example you might have the title bar and the left hand menu in the Master Page. You would then use this master page for every content page in the site. Some advantages are:
You can maintain a constant look and feel between all the pages in your site.
If you need to change any of the common elements such as the title or menu you have a single location to make that change.
You can reuse the content pages wth different Master Pages to change the look and feel without changing the content page.
As Rajesh stated in point 2 you can use them for site customization where a user can select a predefined layout the like.
You can extend that customization to selling predefined website layouts for hosting.
As Rajesh gas stated in the disadvantages is the fact that when migrating existing code to master pages client script and some code may break due to the fact that the controls on the content page are nested differently then they were before.
I hope this helps you.

to using master pages. The master page is merged with the content pages to creat a single unit. In my opinion it is a concept that was sorely missing from ASP.Net 1.x where the common method was to use user controls to achieve a similar effect. The disadvantage is that the user controls needed to be nested in every page and the code was controlled by the containing page. With master pages you can have code in the master page to control its content while the content page can simply manage its content. This gives a clean separation between the site common layout and the content.