The most basic form of filtration, the particle is larger than the space between two fibres, and so, cannot follow the airstream through and is captured.
The mechanism by which large, high density particles are captured is called impingement. As the particulate laden air passes through the filter media, the air tends to pass around the fibres. Inertia of the particulate causes it to separate from the airstream to collide with the fibres to which they become attached.
Diffusion occurs specifically with very small particles which follow irregular patterns, in a manner similar to gases, and not necessarily following the airstream. This irregular pattern is known as Brownian motion and increases the particles chance of capture through contact with the fibres.