Portal is a very interesting concept design and tested by the greatest minds available. The idea of the game is to test the brain by challenging it to think outsidet the box. You can open two portals, the first portal is your destination, the second portal is your entry, or depending on your aiming - they are both your entry and exit. Confused? So you should be. In essence its quite simple, cant get over the gap? Aim, create a portal on the other side of the gap, then create a portal next to you and go through it, you'll be laughing on the other side of the gap in no time! 

Useful Links

Portal: Screenshots
Portal: Videos