In-Line Saw - These are the most common types of circular saws and tend to be the least expensive, also they don't weigh as much as the Worm Drive saws. An In-line saw's motor sits perpendicular to the blade and a shaft runs directly from the motor to drive the blade. An In-Line saw will suite most of your applications and except for changing the blades, they are maintenance free.
Worm Drive Saw - A worm drive is for heavy duty use and tend to cost a little more than the In-Line saws. A worm drive motor is positioned parallel with the blade and uses a gear to increase the torque transferred to the blade. These tend to be a little heavier than the in-line models. Also worm saws are not maintenance free, you will need to occasionally add a special oil to the motor, which is nothing more than unscrewing a screw and adding the oil, it is very easy.

Circular Saw Features
Electric Brakes - This is one feature I would get, no questions asked. An electric brake stops the motor in about two seconds after releasing the trigger, while a saw without an electric brake will take about 10 to 15 seconds to stop. The electric brake works by reversing the flow of electricity to the saw motor. The reason to go with the eclectic brake, which most models now offer, is to save limbs and fingers. After you are cutting for a while your arm will tend to get tired and you want to make sure your safe at all times. Stopping the blade quicker means less time and chances of getting hurt.
Blade Capacity - As noted above circular saws come in different blade sizes. If you are buying a corded model I would go with a 7 1/4" and cordless go with a 6 1/2". The larger blade models are more heavy, but in the long run you will have to make less cuts, especially on angle cuts.
Shaft Locks - Make sure your saw has a lock. A Shaft lock is usually a button you press that holds the blade still, to make changing the blade easier. If your buying a professional power circular saw, it should have this feature. The cheaper models usually do not have this feature.
What to look for when buying a circular saw
I would definitely look for a circular saw that accepts 7 1/4 blades. These are the most common blades; therefore making it easier to buy blades down the road. Also I would look for a saw with a long cord, you would be surprised how many cords are very short. You can always add an extension cord on, but I would look for a cord that is at least 8 feet in length. Another primary item to look for is an adjustable base plate. This is good for making angle cuts and sometimes comes in very handy.
Cordless Saws - Cordless saws are nice because they have no cords to plug in, trip over or even cut through. However they lack the power. The blades tend to be smaller therefore they do not cut as quick. Since it takes a lot of power to cut through wood, you will definitely need more than one battery as you will be recharging them often.
Corded Saws - Corded saws are more powerful, but also heavier. If you are going to be cutting a lot of wood, grab the corded model as it will save you aggravation done the road.
I currently own a cordless and corded one. I find myself using both models just about the same. It is just easier to grab the cordless one and start cutting. Most of the carpenter I know or have talked to also own both styles.