So that's what these are called! I love these things! I've actually heard/read one: The room was draped in gloom and vines. (Courtesy of Brandon Sanderson and Firefight, which I would highly recommend to anyone looking for a new trilogy)
Contractions function almost identically to the full two-word phrase, but are only appropriate in some places in a sentence. It's one of the quirks of this language we've. Some people say English is confusing. I reply, it's.