To turn them into full-time team members in the future.

People who go out of their way to take internships at unconventional companies are disproportionately likely to be awesome team members.

People who call themselves "interns" are much more likely than a "seasoned" pro to expect to spend time learning. The gradually increasing disinclination to learn new stuff is the technologist's apoptosis. It happens to everyone, but makes people who think of themselves as students interesting to work with.

Interns are cheap, but "internship" is so temporary, and the pay scale of an intern has so little to do with their FT comp --- wait, it has zero to do with their FT comp --- that cheapness isn't a meaningful motivator.

We do internships year-round. We don't require people to be currently enrolled to do it.

I'm not a US citizen, but I'm looking forward to apply for internships in the US next Spring.

What are in your opinion the skills that an intern must have to get a position? It is normal in the US to consider foreigners for an intern position?

Different reasons for different people. I really like Fog Creek's take: they hire interns because they believe, for a certain caliber of employee, the internship is likely to be their last exposure to the open hiring market. It is the not-so-secret-secret-sauce for how they identify and attract very talented developers.

http://www.inc.com/magazine/20070501/column-guest.html (This is a reprisal of an old JoS post that I can't seem to Google at the moment.)

I think you are looking for this article:

Finding Great Developers

http://www.joelonsoftware.com/articles/FindingGreatDeveloper...

To do the work that I want to do, or know needs doing, but could never get approved by my management. SOMETIMES it's tedious work that just has to get to do move the project forward, and SOMETIMES it's going to result in significant, measurable benefits to the company. Either way, it's a great way to get to know a developer before officially hiring them.

To do the work that no one else wants to do.

I can't tell if you're joking or serious; I hope it's the former. As an intern, my experience has been exactly the opposite--the first place I worked, I got a very interesting project with almost complete autonomy; the company where I work now I have been able to work on what I'm interested in as well.

One of the good things about working at a startup--even for an intern--is that there is always enough interesting work to go around.

Just a joke, but I'm probably going to get down-voted a million points. Oh well..

I think interns in other fields actually do get to do things that nobody else wants to, which is why it could be unclear. I'm very happy that this is not how it works in software development.

Because they're cheap.