When submitting articles to sites such as ezinearticles.com can I submit content that is on my site or must it be unique. If I submit my article and it gets published, can I then use it on my site
It is better that you modify the article when you publish on your websit, so that you do not encounter Google duplicate content penalty.
I was wondering the same question before I got into article marketing as well, and everyone I asked never gave me a very clear answer. Well here you are, a clear answer: I learned that rewriting or 'spinning' an article before submitting it is what you should do. Do that for each website you submit your article to. The only reason you really need to do that is so Google does not penalize you for duplicate content. Post the original on your site and submit rewritten or 'spun' articles to the other websites. It doesn't need too much of a change, just make sure it's not the exact same article being spread around.
It's best you modify the content. Duplicate content is a bad thing and your article at EzineArticles will out-rank you in the SERPs. I wrote articles on my site and then rewrote them before submitting to EzineArticles. I strongly suggest you do the same or the articles on your site will be pretty much nullified.
Like said before, different content is better to avoid google duplicate content. I also find completly different content is usefull. Once a visitor reach your site from an ezine, you want him to remain and keep him onsite with original content.