Registering multiple domains vs. subdomains

Subdomains have been created to do exactly what you want. If I were you, I didn't buy multiple domains and use subdomains instead. You don't need to spend money for this.

Moreover, buying many domains means working SEO of multiple websites. With only one domain, you can create a community around a website (a brand). And you can link all subdomain to top level domain to build netlinking. For your information, each subdomain is considered by Google as a website.

Otherwise, iframes are bad for SEO, don't use them.

As the other people commented, here are 2 decisions to take:

• "subdomain" (videos.example.com) vs. "URL directory" (example.com/videos) for allocate your "thematic sites" or sub-sites.
• how to manage the "alias" for these sub-sites, as videoexamples.com with an own domain name.

Regarding to first point, i think that there are some proved facts:

• example.com/videos will increase the SEO relevance of your main web project
• videos.example.com will be treated as Google more like a "different" site
• in any case, if you put a different site (CMS) in this thematic site you should do the same SEO work on it independent on the URL structure you choose (content SEO, external SEO, etc..), so think that it's only ONE of several SEO questions, and then is not a so important decision

Regarding the second point:

• i think that's a good idea to register a "nice marketing" domain, as videoexamples.com, and this shouldn't affect SEO behaviour WHENEVER you avoid to offer DUPLICATE CONTENT to Google.
• so, in practical terms, i would define the new domain name as an ALIAS of the thematic URL site, and i would configure server (.htaccess in apache server) for redirect 301 to the thematic site.
• in this way, you could use in your marketing: videoexamples.com, but when the users visit this domain, then inmediatly they woudl be visiting (redirected) example.com/videos. For the bots of Google this would happen in the same way ;)

As a personal comment: you don't must worry about redirections and changes in the URL showed at browser of the users... every day people look less the "address bar" of her browser. In extreme, at the mobile widgets sometimes is difficult to see the URL of the page that you're visiting!