I have a space created to install a built in microwave in my kitchen, can I just put a regular counter top microwave oven in that sapce and have the steel frame around it. Will it ensure proper ventilation and is it safe to use it like this. I want to know the difference in a counter top microwave and a built in microwave apart from the neat look that it givs to your kitchen.

A built in system has:

1) a wall mount designed to hold the unit in place.
2) a ventilation system that can vent outside or back into the room.

Most table top models I know of aren't designed for wall mount and most likely don't have undercarriage ventilation.