I don't know that I've ever heard the term "semi-acoustic" - but an electro-acoustic is simply an acoustic guitar with some kind of pickup/mic attached that can be played amplified.
Yes, they can be played without an amp - they sound like a regular un-amplified acoustic guitar. When you plug them in, they sound like an acoustic guitar - but louder.
There's nothing about them that would *not* make them a good beginner's guitar....just like any other acoustic, as long as you get a good one it'll be fine. The only downside is that they normally cost more than their un-amplified counterparts.