All tires are assigned a speed rating that indicates its maximum driving speed, usually found as a letter symbol on the sidewall after the load index. The speed rating symbol represents the highest speed which the tire can safely perform at.
However, tire speed ratings have less to do with high-speed handling capabilities and more to do with the tire's heat resistance. Driving at higher speeds causes your tires to heat up faster, and overheating can cause fatal damage to a tire. A tire's speed rating reflects its ability to sustain driving speeds up to a certain limit under certain testing conditions without overheating. Each letter corresponds to a certain speed threshold.
The tire speed rating system ranges from A to Z. However, most tires for consumer use are at least S-rated, indicating a maximum driving speed of 112 mph (180 km/h). Passenger touring tires are often T-rated or H-rated, while ultra-high performance tires tend to have a speed rating of V, W, or Y.
If the mph speeds assigned to each speed rating seem unusual, it's because the speed rating system was developed in Europe, so the speed thresholds are set at increments of 10 km/h for the most part. W and Y ratings were added later, and follow increments of 30 km/h instead. (Y)-speed tires put brackets around the speed rating to indicate that the tire can sustain speeds higher than the Y-rating threshold of 186 mph (300 km/h).
You may also notice that, while the speed ratings are mostly in alphabetical order, "H" indicates a maximum speed of 130 mph (210 km/h), between U and V. This is because the speed rating system originally only had three ratings: S for a standard maximum speed of 180 km/h, H for a higher maximum speed of 210 km/h, and V for a maximum speed exceeding 240 km/h. When the rating system expanded to include other symbols, the V rating was changed to mean a maximum speed of 240 km/h, and H was left unchanged.
Z is not included in the chart above because of its unusual usage. Since V was originally the highest speed rating, a "Z" speed rating indicates that a tire can sustain speeds above 149 mph (240 km/h). Z-rated tires indicate a ZR in the size code (for example, P265/70ZR17) and will still have a W, Y, or (Y) speed rating in the usual place.