Micelles are aggregates of molecules of a surfactant in a bulk liquid phase.
As the concentration of a surfactant increases, adsorption takes place at the surface until it is fully overlaid, which corresponds to the minimum value of the surface tension. After this, micelles form in the volume phase. The groups of micelles with less affinity to the volume phase (usually the hydrophobic groups) are oriented inwards.
Micelles in which the hydrophobic group is oriented outwards are described as inverse micelles.
Micelles can incorporate substances which are not soluble in the bulk liquid (e.g. oil in water). The action of surfactants is based on this, e.g. for washing.
The transition concentration at which micelles start to form is described as the critical micelle concentration (CMC). The CMC is an important characteristic for surfactants.