That letter at the end of your tire's size? It represents the tire speed ratings. (If you aren't sure what your tire size is, just check the sidewall.) For example, if your tire size is 225/50R16 89S, the letter "S" would represent your tire speed ratings.
But what does it mean? Tire speed ratings tell you how fast you can drive on your particular tires. Sounds crazy when you look at the numbers associated with speed ratings, but in Europe, some highways don't have speed limits and high speed riving is accepted—though not recommended by the tire manufacturers. Tire speed ratings were established to make sure speed capabilities between tire and vehicle were in agreement. Meaning, the vehicle should only go as fast as the lowest speed rated tire on it.
Read "How to Read Speed Rating, Load Index & Service Descriptions" for more details.