How can we display a simple custom page in SharePoint?

There is no concept of a simple page in SharePoint. There are two types of pages in SharePoint  i.e.: Application page and Site page.
Application page is a generic page while site page are custom to a website. To just cool you off, let us display a simple Application page first.
Some points to be noted about Application pages:

• They are pages which are shared across sites like ‘settings.aspx’, which will help us set generic properties across sites in a site collection.
• The second important part is that we need to save application pages in the ‘C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS’ folder. If you browse to the folder, you will find SharePoint’s own application pages.

OK, what we will do is, to build confidence, let’s make a simple page called SimplePage.aspx. We are not doing anything great in this, we will just write this simple sentence inside the page:

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<b> Ohhhh I never Knew SharePoint is so easy </b>

Once you have saved the page, just browse to the IIS application and browse to the _layouts folder as shown in the figure below. If you open the page in a browser, you should see your display message.