Extra Vehicular Activity -- technical term used in space flight for an astronaut working outside the spacecraft. Used to be just called 'space walk' in early days of space flight when astronaut had no real business (other than floating around for fun) outside the spacecraft.
An EVA is dangerous business for a number of different reasons. The primary one is collision with debris. Orbital velocity at 300km above the Earth (typical for a Space Shuttle mission) is 7.7km/sec. A particle as small as a fleck of paint or a grain of sand has the kinetic energy of a rifle bullet! Every space mission creates more orbiting debris, so this problem will continue to become worse.
Scientists are developing tele-operated robots for outside construction work, to eliminate the need for EVAs. However, the first of these systems won't be ready until at least 2003.