You've probably heard of pressure cookers? They were all the rage until micro wave ovens became popular in the 1980s. They're like over-sized saucepans with lids that seal on tightly and, when you fill them with water, they produce lots of high-pressure steam that cooks your food more quickly .
Autoclaves work in a similar way, but they're typically used in a more extreme form of cooking: to blast the bugs and germs on things with steam long enough to sterilize them. The extra pressure in an autoclave means that water boils at a temperature higher than its normal boiling point—roughly 20°C hotter—so it holds and carries more heat and kills microbes more effectively. A lengthy blast of high-pressure steam is much more effective at penetrating and sterilizing things than a quick wipe in ordinary hot water!