There are many ways to deploy your customizations in SharePoint 2007, for example a custom web part can be easily integrated adding the DLL in GAC and declaring the the control as safe in your web application's Web.Config. Unfortunately this is not the best way to add your customization in SharePoint 2007 because you will never have a clear reference of what have been customized in your farm. The best approach is always to develop a SharePoint Solution, which helps SharePoint to know how to install and/or retract it. The Windows SharePoint Services SDK describes solutions in this way:

The Windows SharePoint Services solution framework provides a way to bundle all the components for extending Windows SharePoint Services in a new file that is called a solution file. A solution file has a .CAB-based format but a .WSP extension. A solution is a deployable, reusable package that can contain a set of Features, site definitions, and assemblies that apply to sites, and that you can enable or disable individually. You can use the solution file to deploy the contents of a Web Part package, including assemblies, class resources, .dwp files, and other package components. (read more).

In this page, you will learn how to create a Solution based on a Site Definition using SharePoint Solution Generator.
First of all, create a new Site Collection (Central Administration > Application Management > Create Site Collection) selecting the Team Site Template. In my case i will use an existing Web Application located on http://myro-win2k3:90/ . Press the Ok button and navigate to your new site collection.
Once you are in, lets make some personalization:

• Navigate to /_layouts/create.aspx
• Create a custom list named "Sample List" 
• Add a column named "Sample Column", selecting "Single line of text" as type.
• Navigate to /_layouts/create.aspx

Now that we have a new Site with some customizations we can open SharePoint Solution Generator to create our new custom Site Definition,  based on this site.

• In the first step, choose Site Definition
• next, specify your site's url ( in my case is http://myro-win2k3:90/default.aspx )
• then check just Form Templates and Sample List in a Pick a SharePoint List step
• Specify save location writing "mySolution" as Project Name
• Check if the target site is correct and press Finish

Wait untill Solution Generator completes the task and then open the mySolution.csproj with Visual Studio 2005. The first thing we must try is to deploy the solution in SharePoint.... Click Build -> Deploy. Automatically Visual Studio will try to deploy it in http://localhost/ but if you need to deploy it on a different site click Project -> mySolution Properties -> click the Debug tab -> and set your target site in Start Action -> Start Browser with Url.
If your deploy is succesfully you can create your first site based on your Custom Site Definition, but where is the fun? This Site Definition is not yet custom, till we don't customize!

Let's try out:
The Form Templates is a List Definition of a Document Library and this folder contains all the ASPX pages needed for your customizations. Open the DispForm.aspx under Form Templates locate on top the PlaceHolderPageTitleInTitleArea PlaceHolder and add this line:

t's easy to understand that customizing this pages you can modify almost everything. We have just added some text in the DispForm.aspx, infact if you add a new document to this library and view the properties (DispForm.aspx) you see the changes you made before: