>what is a good boom mic?

Any mic that you put on a boom pole, usually a shotgun (narrow pick up pattern).

>Is a shotgun mic a boom mic?

Could be.

>and is the Sennheiser ME66 a good mic (is it a shortgun or shotgun?
Is there a difference?)

The Sennheiser ME66 is a short shotgun. A ME67 is a long shotgun.
They have "adequate" sound quality, but not great.

The next step up the food chain is the Sennheiser 416, which
is a "short shotgun" that actually has a lot of reach. Very good sound quality.

I was told that very Clint Eastwood movie used a Schoeps MK47, which is a small
hyper cardoid that fits in a blimp about the size of softball. Excellent sound quality.

You should know that Schoeps mics, though very very good, also are among
the most temperamental when it comes to moisture, followed by Neumann
and Sennheiser. When I priced an MK47 "kit" it was about $4300 for
the capsule, the power module, the preamp, the Y cable, the blimp, furry, etc.

Thanks Jacques for the info. I'll probably shoot indoors mostly. Is that okay with the ME66? Thanks for the help again.
Neil