What’s the difference between “use” and “intent to use” trademark filings?

As the name implies, a filing based on “use” means that you are currently using the mark on your goods or services in commerce.  “Intent to use” means that you have a “bona fide intent” to use the mark in commerce at a later date.  The USPTO allows you to file a trademark application based on intent to use as long as you genuinely intend to start using the mark.  The USPTO cannot grant a registration until you submit proof of use, but will grant you up to 5 extensions of 6 months each to give you time to start using the mark in commerce.  If you cannot show use of the mark within that time, the application will be deemed abandoned.