2 Answers

The R16 is the size, diameter in inches, of your rims. So you need a tire that fits a 16" rim if you are just replacing the tires. The "225" indicates the width of the tires in millimeters and the "45" is the heigth to width ration (aspect ratio) in millimeters of your tires. For a complete diagram of what everything listed on your tire represents check out this diagram.
http://img196.imageshack.us/img196/5279/tiresidewall.gif

It sounds complicated but it's pretty straightforward and easy to understand when you know what it all means.Basically it's the important dimensions of your tyres that you really should know.
Ok 225/45 R16.
The 225 refers to the width of your tyre - if you imagine standing at the front of your car looking at the tyre front on then thats thd width,thou more specifically it's the tread width and in your case it's 225mm or 22.5cm wide.Now 45 refers to whats known as the aspect ratio meaning the ratio of the sidewall of the tyre (thats the part that has all the lettering inc 225/45/16) but more simply its a percentage i.e the height of the sidewall is 45% of the tread width or 225mm which is 101.25mm.R stands for radius and 16 for 16 inches which is the radius of the wheel it fits.
So your tyre basically has 16 inch radius with 225 mm of tread width and a sidewall/height of 45% the tread width