i saw whats called a set screw fitting, you tighten the screws to lock down the coupling. there only supposed to be used indoors. when there is a heavy dew, moisture will condense on the conduit, then run down any vertical piping you have... when it gets to the set screw fitting, some of that water will work its way inside the pipe, the wiring will be unaffected. but the
conduit is only galvanized on the outside... let water get to the inside, and it will rust out faster than an old buick.
a compression fitting uses a ring seal to make a watertight connection for running EMT (thats the type of pipe your using) outside. it looks like this: