Attic insulation is measured by Resistance (R) to the flow of heat. Different materials provide different insulation capacities. General, one inch of insulation will provide an R value of 2-3 units. So, six inches of insulation will provide a rating of about 12 to 18.
Insulation is usually fiberglass or cellulose. In newer homes, there may be foam insulation (open or closed cell) sprayed into wall or attic spaces. It is a good insulator and requires less labor to install, but it can have other problems.
A vapor barrier is used to keep humidity/condensation from affecting the insulation or from gathering on walls and ceiling surfaces adjacent to living spaces. In very humid climates, a vapor barrier can actually cause problems and should not be use.