Passenger Tire sizing is typically displayed as:

P205/65R15 89H
The P stands for "P-Metric" or "Passenger". Tires with higher ply ratings will generally start with" LT" which stands for "Light Truck". This indicates the tire is an LT metric and will always have a Load Range indicated. This is important for all vehicles calling for LT tires. Never substitute a P metric tire for an LT metric tire, even if all the dimensions are the same.
The "205" is the width of a tire, also known as the "section width". This is the width of the tire in millimeters at its widest point from sidewall to sidewall when mounted on the recommended rim width. The actual tire width can vary depending on the rim width it is mounted on.
The "65" is known as the Aspect Ratio. It is calculated by dividing the section height by the section width and multiplying by 100. (In this example, the sidewall will be 65% of 205).
The "R" stands for Radial, meaning it has a radial construction. Radial tires have ply cords that extend to the beads and are laid at 90 degrees to the centerline of the tread, the carcass being stabilized by a circumferential belt. Other possibilities include "D" for diagonal construction. This means the ply cords extend to the beads and are laid at alernate angles less than 92 degrees to the centerline of the tread.
The "15" stands for the diameter of the wheel in inches. This is the exact size that this tire will fit.
The "89" is the load index. The load ratings for any tire (load index) indicates the maximum weight that each tire is able to support. For example the "89" will support 1278 pounds per tire. The higher the load index the more weight it will carry.
The "H" is the speed symbol. The speed rating of any tire is a measurement of the top safe speed the tire can carry a load under specified conditions. It is also an indication of how the tire will handle a lower speeds. A higher rated tire will give you better traction and improve steering response even a 50mph. The "H" rating top safe speed is 130 MPH. It is not recommended to downgrade your tires to a lower speed rating of the tires that's recommended for your vehicle. Doing so can effect the handling of the car, and can be dangerous. Also,never mix tires with different speed ratings on your vehicle.
Some other markings you may find on your tire's sidewall:

The M+S indicates the tire has all season capabilities. You will find this on all season and winter tires.

Dot stans for the Department of Transportation. The 10,11 or 12 digit code appearing after the DOT designation gives information such as the week and year the tire was produced, as well as sthe manufacturer, plant, tire line, and size.

Maximum Inflation Pressure is the highest inflation pressure that the tire can withstand. This is not, however, the recommended inflation pressure. Inflation pressures should never be below the recommended pressure or above the maximum pressure branded on the sidewall.