My specialty!
Ok, on my car i have:
P205/65 R 15
p = "Passenger". its the North American tire sizing designation sometimes you'll see like T, or LT for trucks or light trucks.
205 (width of tire) also known as the "section width". What this is, is the width of the tire in millimeters from the widest points. (sidewall to sidewall).
65 is height also known as " Aspect Ratio". Actually techniclly its calculated by dividing the section height by the section width and multiplying by 100. (In this example, the sidewall will be 65% of 205)
R means that its a radial tire. Thats how its constructed.
15 = rim size. 15 inch rim.
Sometimes you'll see like a P infront of it
Then theres the speed ratings. Different Letters mean different speed ratings. Dont know them off the top of my head. Z rated tires are rated over 150 mph. Dont know if there the top or not.