In Research Universities, there are many ways to rank scientists:
(1) Undergrads: Some perform research. Some of this work gets published in science journals or in Undergrad Theses.
(2) Graduate Students: Most perform research. This is probably the heart and soul of Academic Research. Graduate students perform most of the work, write the papers, and write MS and PhD Theses.
(3) Postdocs: They train Grads and Undergrads. They write grants. They perform research and write papers. They are experienced researchers, but they typically have short appointments and many hoops to jump through to maintain these appointments or line up their next one.
(4) Professors: They have many duties including supervising groups (1-3) above, teaching classes, reviewing grants and papers, writing grants and papers, promoting the work done in their lab, etc. Assistant Professors are under the gun to set up a lab and prove themselves in order to get Tenure.
(5) Professors who are Principal Investigators: Every grant has at least one Principal Investigator. Usually they are Professors, but some might be Postdocs. The PI is like the CEO for the grant.
(6) Nobel Laureates: These folks are definitely scientists.
There are also a variety of Technicians, like those who work in the Machine Shop, fix Electronics, keep the computers running, maintain colonies of rats, sequence DNA, run Histology Labs, etc. These people don't always get listed as authors on papers, but they are vital to the work that gets done.
Some common measures of a scientist are how many peer-reviewed articles he/she has published, the quality of the journals these articles appear in, and how many times these articles have been cited by others. Book chapters, books, invited talks, and presentations at conferences count too. Perhaps nowadays, YouTube videos, Ted Talks, and the number of views/likes these get also count.
I'm not sure how it works in Medical Schools, Hospitals, National Labs, or the Corporate World.