Electron Cloud
The electron cloud is the region around the nucleus where an atom's electrons are likely to be found, replacing the idea of fixed orbits.
Definition
The electron cloud is the fuzzy region around the nucleus where an atom's electrons are likely to be found; electrons move so fast and unpredictably that we picture them as a cloud rather than as dots on fixed paths. The cloud is denser where an electron is more likely to be, and it is arranged in energy levels with the valence electrons in the outermost one. In the quantum mechanical model each electron is described by a wavefunction $\psi$, and $|\psi|^2$ gives the probability density of finding it at a point; orbitals ($s$, $p$, $d$, $f$) are the shapes that enclose most of that probability, and the Heisenberg uncertainty principle forbids assigning an electron a definite trajectory.
Example
A fan blade spinning fast looks like a solid blur, and the electron cloud is that blur: the electron is in there somewhere, but you cannot point to one spot. In hydrogen the cloud is a sphere about $0.1\text{ nm}$ across, with the electron most likely about $0.053\text{ nm}$ from the nucleus. A $1s$ orbital is spherical while the three $2p$ orbitals are dumbbells along the $x$, $y$, and $z$ axes, and carbon's bonding in methane, $CH_4$, comes from mixing its $2s$ and $2p$ clouds into four $sp^3$ hybrids pointing to the corners of a tetrahedron.
Key Insight
Electrons do not circle the nucleus like planets around the sun; the Bohr model survives in textbooks because it is easy to draw, but it fails for any atom with more than one electron. The cloud sets the size of an atom, so when two atoms bump, cloud touches cloud, and orbital shapes rather than orbits are what predict molecular geometry.