The player controls Chell from a first-person perspective while running and jumping to completing various puzzles. She is able to withstand large drops, but can be killed by falling in the toxic water of the facility, crushed to death, burnt to death, or shot to death by turrets.
The game is generally divided into a series of test chambers, though it also features other sections that are more exploratory.
The Goal
In the test chambers, each chamber has an exit door that must be reached, often requiring that certain conditions have been met such as having weighed down a large button with a Weighted Cube. These puzzles require the use of the Aperture Science Handheld Portal Device, aka the portal gun.
Portal Gun
The portal gun can shoot two portals, colored differently for identification, on any flat surface that can accept a portal. Once both portal ends are placed, the player can walk between them. Portal ends can be re positioned as often as necessary.
It's the only weapon you get in the game and you must use it to get around obstacles, such as walls that don't let you shoot portals at them, automated turrets that you have to knock over.
Momentum
A key feature of the game is that momentum is maintained when traveling through 
portals. When portals are placed on non-parallel planes, this can create the effect of "flinging", allowing you to gain speed and distance that normal jumping and running could not generate. A leapfrogging effect can be used by placing portals in series during this flinging, gaining further momentum with each use.
As GLaDOS puts it, "In layman's terms: speedy thing goes in, speedy thing comes out".
