The contracting company that wins a bid for a contract is the Prime contractor. Often times these companies will then subcontract out some of that work (and sometimes it's even required by the contract to do so) to other contracting companies which would then be the Sub-contractor. That's where the "Prime" and "Sub" lingo in re: to DoD contracting comes form.
And the sub-contractors are sometimes further broken down into Tier 2, Tier 3... depending on how far down the value-chain they are
Prime Contractor.
It is a company that is responsible for the final integration of a system, but often fans out subcomponents to a large assortment of other companies.