The crux of the gameplay involves going between portals. Blue and orange portals connect each other, and players can walk through portals (fired anywhere onto walls, floors, and ceilings) to get to each level's exit. This bewildering mechanic is introduced slowly, at first involving pre-placed portals, then giving the player a portal gun that can fire one type of portal, until finally allowing the player to manipulate both portals at will. Much of the puzzles involve "simply" spatial navigation, although even that can be difficult to figure out. Later levels will add switches and blocks, traps, enemy turrets, elevators, and momentum.
Portal is one of the first few games that actually utilize a basic construct of physics - momentum. By falling far enough, you will gain momentum, and thus - speed. This extra speed, when combined with additional portals to let the player freefall for longer periods of time, will let you fly across wide gaps. Whenever you figure out a momentum puzzle, it is both intellectually satisfying and physically exhilarating as you zoom across the room.