The bead of a tyre is the part that anchors it to the rim when fitted. Historically it is made of several strands of high tensile steel wires, although some manufacturers are making the bead out of a single strand of wire wound several times. This single strand manufacture tends to give a more even construction resulting in better tyre uniformity.
In the past tyre beads have been round in shape. Some manufacturers have redesigned the bead to ensure better fit the rim, resulting in a diamond-shaped bead.
<pDiagram shows cut away of a tyre showing a typical car tyre bead construction