Terminology 101 for newbies. The government buys like a big company sometimes, but often they buy like a small company. Realize it is not a big company. Doing business with "the government" is more like thousands of little, medium and big companies.
So, some parts of the government refer to the same thing different ways. Semantics matter. An example of that is Indefinite Delivery Vehicles (IDV). The website www.usaspending.gov is a great resource. References there are to IDVs. For those of you accustomed to commercial business, an IDV is just an open purchase order a company may have with a supplier or service provider. The agency is basically establishing, through a process, competitive prices and terms and conditions with a company or group of companies. The award of an IDV doesn't necessarily mean any funds are actually awarded though. Task Order/Delivery Orders/Purchase Orders would follow with the actual funding.
GSA is a good example and usually the first encountered in government purchasing. A GSA contract comes with no orders. The newly awarded contract simply establishes your pricing for specific items and gives you terms and conditions (payment terms, reporting requirements, points of contact, etc). Separate, individual task orders would need to be competed once you got a GSA contract.
IDV is also known as Indefinite Delivery/Indefinite Quantity (ID/IQ). There are also Multiple Award Contracts (MAC) which just means multiple companies were awarded a seat at the table. Government Wide Acquisition Contracts (GWACs) are just really big IDVs where all government agencies can purchase using the contract.
Bottom line, a government worker may walk up with a credit card, but more frequently a contract will be competed. Often the IDV is just the first round of competition then another round of competition would actually result in funding.