My understanding is that it's a more complex process, but once you're processed, it's much faster to get through airport screenings in future. Is it available to non US citizens as well, and if so, once approved, how long are you approved for?
Yes and No.
There are a few different ways to get TSA Pre - each with it's own restrictions.
As an Australian with high frequently flyer status with United Airlines I get Pre access the majority of times I fly (and have pretty much since the Pre program was introduced).
Short answer: Yes, if you're a US permanent resident or a citizen of the Canada, Netherlands, South Korea or Mexico, and apparently it's valid for five years.
And while their link is broken, the trail of breadcrumbs leads to the CBP: