The term load testing or stress testing is used in different ways in the professional software testing community. Load testing generally refers to the practice of modeling the expected usage of a software program by simulating multiple users accessing the program concurrently.
Historically, all load testing was performed with automated API tests that simulated traffic through concurrent interactions at the protocol layer (often called protocol level users or PLUs). With the advance of containers and cloud infrastructure, the option is now present to test with real browsers (often called browser level users or BLUs). Each approach has its merits for different types of applications, but generally, browser-level users will be more akin to the real traffic that a website will experience and provide a more realistic load profile and response time measurement. BLUs are certainly a more expensive way of running tests and cannot work with all types of applications, specifically those that are not accessible through a web browser like a desktop client or API-based application.