Galileo Galilei first discovered Europa and the other prominent Jovian moons in 1610; it was the first time a moon was found orbiting a planet other than our own. This discovery was an important factor in revolutionizing our understanding of the Solar System's dynamics, and discovering that our Solar System revolves around the Sun rather than the Earth. Perhaps one day these moons will be a part of a new revolution, where not only are there other planetary moons, but also other worlds that harbour life.
Like Enceladus (discussed below), Europa is covered by a global ice sheet, and it is thought that underneath this sheet lies a deep ocean of liquid salt water. Europa is too far from the sun to form liquid water from solar energy alone; however, it's eccentric (or stretched) orbit means that it is closer to Jupiter at certain times and farther at others. This change in the gravity experienced by Europa causes it to stretch due to gravitational tidal forces, which in turn warms the subsurface ocean and likely causes many of the cracks that are seen on the icy surface.
The warming is also the source of speculation that the ocean may be habitable for simple microbial life. However, many scientists believe that although the ocean would have enough chemical reductants, it would not have enough oxidants for complex life to emerge (life is presumed to require a supply of both to survive). Earth’s ocean gets its oxidants mainly from interactions with the atmosphere. This process is simply not possible on Europa given the thick ice sheltering the ocean from above. However, models have been proposed where charged particles from the sun create oxidants at the ice surface which are able to migrate to the subsurface water below. It seems very possible that the right ingredients for life may exist on Europa, but perhaps not enough for complex life to form.