Tyres have a code system moulded into their sidewall which allows you to understand their technical capabilities. This code provides information on the tyre's construction (e.g. radial), its size, its load-carrying capacity and its speed rating. For example, the code on a common fitment for Australian cars is:

205/65R15 95H

205 indicates the nominal section width of the tyre in millimeters (205mm).
65 indicates its aspect ratio, a comparison of the tyre's section height with its section width (65 indicates the height is 65% of its width).
R indicates radial ply construction.
15 indicates the nominal diameter of the wheel rim (15 inches)
95 His a symbol indicating the maximum load capacity and speed at which the tyre can be safely operated, subject to the tyre being in sound condition, correctly fitted, and with recommended inflation pressures (95 represents a maximum load of 690kg per tyre; H represents a maximum speed of 210km/h).