A lab is, and should, never be stagnant. I'm not talking about research, though this is constantly going and projects evolving. No, this post is about the cast of characters in a lab. There are several different types of characters that can grace the lab of a research based graduate program.
From top to bottom of the food chain we have
PI - Principal Investigator, the boss, the big Kahuna, when they want to work in the lab you get out of the way
Postdoc - Have their doctoral degree but not quite ready, or desiring, to run their own lab. Advantages of postdoc are often no teaching which means lots of research time, disadvantages potentially more admin/managerial roles in the lab.
PhD Student - Crazy people like me. We can be juggling teaching, courses, research, studying, writing, all at the same time.
Master Student - Also juggling teaching, courses, research, studying, writing but for only 2 years
Visiting Scientist - This is a short term, weeks to months, situation where a graduate student from another institution that wants to learn new techniques that your lab is particularly awesome at (or at least decent and willing to take a visitor). I only put them below the other grad students as they won't know the labs procedures and are there for training.
Interns - Undergrads doing research in the lab. Often they are juniors or seniors getting their research credits.
Freshman workers - dishwashers, tip rackers, anything everyone above does not have time or desire to accomplish.
Other than the PI, the individuals filling these roles flux on a semi-regular basis. The start of a fresh semester brings with it new schedules and, often,
new people. This is particularly true of the fall semester. Fall is the most common time for new graduate students to start, and is always the time that new freshman workers start. Interns can come and go each semester. Grad students and postdocs tend to stick around for a few years.
This semester, our lab has picked up a new freshman worker, intern, and a visiting scientist. We also lost our senior PhD student to a Postdoc at a far away lab and one of our MS students will be finishing this fall. It never stays the same long.