Using a 100-watt lightbulb in a lamp with a 60-watt maximum wattage rating is not recommended; doing so is a fire hazard. The maximum safe wattage rating for a lamp is determined by the amount of heat the wiring and fixture can withstand. The higher the wattage, the higher the heat.
A 100-watt lightbulb draws a significant amount that can then overheat a 60-watt lamp. Compact fluorescent lights (CFL) or LED bulbs marketed as having a 100-watt equivalence are safe to use in 60-watt lamps. For these energy efficient bulbs, the watt equivalence refers to lumens, or light output, and not actual power use. A CFL with a 100-watt equivalence is as bright as a 100-watt incandescent lightbulb, but draws approximately 23 to 30 watts of power.