As Timmo said. Square ones usually have movable grilles so you can set the direction of air-flow as it exits the vent. Round ones are better for heating systems.
As to vent positioning, you want to give thought to the air flow. When running a swampy you should have a couple of windows or doors open - a correctly sized swampy will replace all the air in the house in only a matter of minutes - so if you
don't open windows they're trying to pressurise the house and don't work very efficiently. Plus they're more likely to cause condensation on your walls in a "sealed" house.
Hint: If you can open the front/back door just a few inches and it blows shut again, then you don't have enough windows open.
You should position the vents so the air will flow across the room and out the window/door. A common location is near the inside wall of a room, usually close to the door.
Of course, there are people who live a rather sedentary life-style; sitting at a desk or in front of a TV all day. For them, it's usually a good idea to have one vent cooling just that position...