What makes a fixie with a rear freewheel different from other types of fixed-gear bikes?

I have noted the difference between a freewheel and a fixed gear cog in the following table.

A cyclist on a single speed bike with freewheel installed should not have any problem coasting on flat surfaces and cycling uphill.

Without the freewheel mechanism on the fixed gear cog, the cyclist may have to stand on his pedals and strain to pedal uphill. To stop the bicycle from rolling backwards down the slope, the cyclist will have to apply a braking force with his legs and bodyweight to resist the rotation of the cranks.

If your rear wheel is installed with a double-sided or a flip-flop hub, you do not have to remove the freewheel in order to install the fixed gear cog, or vice versa. In fact, some fixies with flip-flop hub on their rear wheels are supplied installed with both the freewheel and fixed gear cog. Cyclists can decide on which of these gears they want their chains to run on. 

You can see what I mean by double-sided hub when you look at the wheel in the following video. From it, you can also learn how a fixed gear cog is installed.

Just send your fixie to the bike shop if you do not have the tools required, to install a fixed gear cog.

Watch this video to see how, when you are cycling on fixed gear cog, you can switch over to cycle on your freewheel or vice versa.