Welcome ads typically appear before a visitor lands on a specific Web page. This ad is typically the primary content on a page, though it may be accompanied by a welcome message as well.
Forbes.com is perhaps the best-known user of a welcome ad; anyone trying to navigate to this site, including the home page or specific sub-URLs, will first see a whole page welcome ad:
This welcome ad has a separate URL (you can see the Forbes one here) and in some cases will automatically forward to the relevant page after a few seconds.
While welcome ads are traditionally used at the beginning of a session on a website (hence the “welcome”) some sites feature them at other points in the process. Below is an example from the online version of Merriam Webster Dictionary; when clicking on a term, visitors are first directed to an intermediate page with a partial definition and a prominently positioned ad:
From this page, a visitor will have the option to continue on to the full definition.
The advantages of the welcome ad are pretty obvious: it is a high impact ad unit that can generate incremental display ad revenue with minimal cannibalization of existing ad units.
The potential drawbacks should be pretty clear as well: the welcome ad may annoy some visitors, leading to fewer pageviews and return visits. As with many high impact ad units, frequency capping of a welcome screen is a must; visitors should not be exposed to this ad unit repeatedly.
Welcome ads may also impact the SEO visibility of your site. The properties that regularly use welcome ads tend to be older, established properties who have established a reputation for quality with search engines. Younger, smaller sites could potentially see the negative side effects of welcome ads (specifically, a high bounce rate) have an adverse impact on search engine visibility.