Most have an optional charcoal filter that you can use if you choose the recirculation installation. In the OTR microwave we just installed, it came out of the box ready to vent out the back (for a vented application), but we had to open it up and rotate the fan so it would recirculate instead.

An easy method to make sure you are getting one that can vent properly for your application is to check out the unit's dimension guide on ajmadison's website. If you see one that says "Convertible to recirculating", it likely comes ready to vent outside unless you switch the fan.

As for your other question, the fan function is separate from the microwave. Just like the light function is separate. There are generally microwave controls, then under that pad you will see the buttons for the fan & light.

The ones I've seen have the charcoal thingy in them but still can vent to the outside. Just remove the charcoal thingy and flip a vent plate around. And the fans are separate. At least on my friend's Kenmore and my mom's Maytag.

Thanks for the responses. I'm so not mechanical that unless descriptions use one syllable words to state exactly how something works I have difficulty understanding what they mean. Now I just have to find something in the right price range. Thanks, again.