Quality Assurance is the proactive program of policies, procedures, training, and audits designed to make sure work meets contract specifications the first time. QA covers: hiring qualified subs and tradespeople, training crews on specific assemblies, pre-installation meetings before each trade starts, mockup approvals before production work, supplier qualification, and process audits. QA is upstream of the actual work.
Quality Control is the downstream complement: actually inspecting and testing the work as it gets installed. The two together (QA/QC) form a complete quality program. On commercial projects, the QA/QC plan is a deliverable. On smaller residential and light commercial work, QA often happens informally through the superintendent's judgment and crew-leader experience. Either way, the contractor with a real QA program produces fewer punch-list items, fewer warranty callbacks, and stronger owner relationships, which all show up in profit margin.