If I place content on a page through an iframe (the source is on my server) is that content seen by search engines and indexed?
Is it indexed as a separate page for my site? If so, what page on my site does google think it belongs to?
You see mix results from using iframes on your web pages, and none will be benifical. You should consider using SSI to inject the content instead, that is the seo friendly method I use.
SSI is a server side include file, I think it's a pretty old technology though. If you don't have a lot of programming knowledge, just put the content in regular HTML. That's even easier to index. Don't use iframes, unless it's for irrelevant content, like polls/results. You want your content to be in context with your website, not on some other page all by itself. This content also won't have the links or other keywords that you want.
As gummybear said, SSI are known as Server-Side Includes. Note that you can use PHP or another server-side programming language to do the same job. In fact, there's a tutorial in my signature that shows just how to do that (with both PHP and SSI). It's the second link.