The electron cloud of an atom is the area outside an atom's nucleus in which electrons exist. The Bohr model of the atom, popular in the early 20th century, was disproven by quantum physics theories put forth by Werner Heisenberg.
Previously, atoms were thought to be like miniature solar systems, with the nucleus acting as the star and electrons orbiting as planets. However, the Heisenberg Uncertainty Principle states that it is impossible to know both the exact location and velocity of an elementary particle; it is only possible to measure either one or the other. This is because electrons are so small that any particles used that interact them cause them to change velocity via physical collision. Velocity refers to both the speed and direction of movement, not just the speed, so the effect would be similar to a billiard ball ricocheting off another one.
Scientists can guess the rough distance between the nucleus and a given electron shell. Rather than orbits, the shells represent a spherical area in which the electrons are located. However, the exact location of the electrons within the cloud is completely random and unknowable with our current instruments. The electron cloud is also called a probability cloud.