215/35 R18Z. For example, is my tyre size.
215 is the width of the tyre, looking down. So the amount of tyre on the road. This is measured in mm, millimetres.
35 is the height of the tyre, looking from the side. This is measure as a percentage of the width. Low number = low profile.
R = radius
18 is the radius of the wheel the tyre will fit, in inches.
Z is the speed rating. Mine is quite high.
Some tyres may have M and/or S, which stands for Mud and Snow.