as jay said, '185' is the width of the tyre in mm, '55' is the percentage of the width for the height, ie 55% of the width 185mm is 101.75mm high. and the final number '15' is the diameter of the wheel.
the letter following the three numbers is the speed rating, i know 'v' meand safe upto a speed of 149mph, and 'h' is upto 130mph.