Yes, it can be plugged into an amp. My friend has one, I wasn't aware it could be plugged into the computer though.
Ibanez GSZ120
Mitchell MD-100S
Roland Cube 60
Boss DD-7
i believe you can plug it into both, 1/4" for the amp and 1/8" for the computer.
I think that one got an USB cable so it can be plug in comp and you can use it in your guitar 2... So i'll say both, but i doesn't have that one exaclty so i ain't sure...
1) that is absolutely not a stupid question
2) it has both a usb out and 1/4" out so will go into an amp and a computer
3) i can;t guarantee it'll go into both at the same time though
The only 6 words that can make you a better guitarist:

Learn theory
Practice better
Practice more
Another way to look at it is.... The RP250 acts as a USB sound card and your guitar amp is the powered speaker.