Basically you build space ships with stages and whatnot and launch them to see how they do, and not much else, but it's in alpha, so that'll probably change.
Quote from website:
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Been playing this for a month now. Pretty fun.

Highest speed I had was around 3000 metres a second.

I got up to 25500.1 m/s, plus the speed stayed there till I ended the flight.

Any successful orbits?

Seems the download page is broken, mind uploading it here? I would like to try this.

They have a mediafire mirror already, which was on the download page.
http://www.mediafire.com/?czxt6h8z56sg0bd

My best ship had a maximum velocity of 3.666 Km/s and the maximum distance 7321Km, after that I stopped the game, but since my velocity exeeded by far the minimum escape velocity of the planet, it would never have come back.
Edit: this is without using any mod.

This game is cool, but the real fun is mods. I love the Orky Bitz mod on their forums.

Highest I got was 1.8 mil with 1400 m/s(km?) it started slowing down though. I just couldn't be bothered to see if it would actually come down or not.

Whelp, using modded parts I was able to attain a steady speed of 27754.8m/s, being too far away to be affected by Kerbal's gravity.

I left to make a sandwich and the game blackscreened.

Making ships that go ridiculously far and fast is easy - I got bored of that, then tried to put my ship into orbit. First attempt, I made it halfway around the world. Then, I think it survived two and a half trips or so. Third time was the charm, and I even had enough fuel to de-orbit.

Is it possible to land and re-launch, though? 'Cos I want to try and make a ship that launches into orbit, de-orbits, then gets back into orbit after landing.

Just don't land on the dark side of Kearth.

There's a dark side? Not for me, although A) I was on me mammy's laptop so it was at lowest settings and B) it was an older version. Haven't updated it.

well, if you land on the opposite side of the planet from where you start, your rocket immediately explodes.

Well, the game is ridiculously updated. Most importantly, Kerbin now orbits Kersol, so you can leave Kerbin and orbit Kersol instead. And orbiting Kerbin is the Mun, which you can land on/orbit as well. Also, there's now an in-game realtime orbit map that shows your trajectory based on current velocity, it makes Munar landings/Kersolar intercepts much, much easier.

I'm not sure if this was in the older version, but there's also fancy stabilizing winglets/engines, and for orbital reorientation you can use RCS thrusters.

Finally completed my first Mun landing, sort-of. Depends on if you count crashing into the Mun's surface at 200m/s as a landing or not, however.