Benchmarking is the process of running tests on hardware and/or software to assess the relative performance based on set criteria (relating to a project).
Hardware
There are applications which run tests on specific computer system hardware components which generates a comparative report that can be assessed again other similar components. Common components tested for speed are CPU, RAM, GPU (Graphics card), and SSD.
Software
While speed is the common benchmark comparison for hardware, software is also benchmarked on functionality, installation size, accessibility, and security.
Operating systems
Different operating systems will offer varying performance characteristics based on the type of OS and purpose it is to be used. For more see roles of an operating system.