They form because of the segregation of these lipids into a separate phase, demonstrated by Biltonen and Thompson and their coworkers. These microdomains ('rafts') were shown to exist also in cell membranes.

What forms a lipid raft?

Rafts are small platforms, composed of sphingolipids and cholesterol in the outer exoplasmic leaflet, connected to phospholipids and cholesterol in the inner cytoplasmic leaflet of the lipid bilayer. These assemblies are fluid but more ordered and tightly packed than the surrounding bilayer.

What is a lipid raft and what is its function?

Lipid rafts are plasma membrane microdomains enriched in cholesterol and sphingolipids that are involved in the lateral compartmentalization of molecules at the cell surface. Internalization of ligands and receptors by these domains occurs via a process defined as raft-dependent endocytosis.