A splash page is an overlay page that appears on top of your website. It is used to show ads, an introduction to the website (flash animation usually), or improve conversion (example: requesting a subscription to a newsletter or selling a product). If its a user experience site such as gambling or online games it can be used to inform the user of what kind of web browser or software is best to get the most out of the site.
When you create a splash page, you should be sure that search engines don't crawl and index the content of your splash page instead of your main page. How do you do this?
Don't use a pop-up or a specific URL for your splash page, instead, use an Ajax overlay.Tips
Unless your business is well known or you are a content rich website, it is not a good practice to sell something on a splash page. If you want to show promo codes or special discounts it is best to give the users some time to browse through the site and use a pop up window to give those offers.