Diffusion
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until they are evenly spread out.
Definition
Diffusion is the way particles spread out from a crowded place to a less crowded place, on their own, without any pushing or pumping. It is the net movement of particles from a region of higher concentration to one of lower concentration, driven by their random motion, continuing until the concentration is the same everywhere even though the particles keep moving; in cells, small molecules such as oxygen and carbon dioxide cross the membrane by simple diffusion at no energy cost. Fick's first law describes it: flux is proportional to the concentration gradient, $J = -D\,\frac{dC}{dx}$, where $D$ is the diffusion coefficient. The characteristic time to diffuse a distance $x$ scales with $x^2 / D$, so doubling the distance quadruples the time. Across a membrane the rate also depends on the solute's lipid solubility and size, and polar or charged solutes need channel or carrier proteins (facilitated diffusion), which is still passive because they move down their electrochemical gradient.
Example
If someone opens a bottle of perfume in one corner of a room, the smell soon reaches the other side because the perfume particles diffused through the air. A drop of food coloring in still water spreads until the whole glass is evenly tinted, and warmer water speeds this up because the molecules move faster, finishing in minutes where cold water takes much longer. Oxygen in water has $D \approx 2 \times 10^{-9}\ \text{m}^2/\text{s}$, so crossing a $10\ \mu\text{m}$ cell takes on the order of $x^2 / (2D) \approx (10^{-5})^2 / (4 \times 10^{-9}) \approx 0.025\ \text{s}$, but crossing $1\ \text{cm}$ would take about $7$ hours, which is why tissues need capillaries within about $100\ \mu\text{m}$ of every cell.
Key Insight
Diffusion is how oxygen gets from your lungs into your blood and from your blood into your cells; no part of your body has to push it. It is fast over tiny distances and very slow over long ones, which is why cells are small and why your body needs a circulatory system instead of relying on diffusion alone. Diffusion is driven by entropy, not by any force pushing particles apart: each particle is equally likely to move in any direction, but there are more of them on the crowded side, so more cross toward the sparse side than back.