Runner
It is an entry level position which will act as a general assistant, completing whatever basic tasks are required to ensure the smooth running between each departments. Sometimes, it is an opportunity to be an runner to learn about how those positions work in a VFX studio, and it’s a good route to get into the industry for someone without relevant experience.
A runner needs to do a variety of jobs, the following image shows what kind of jobs require a runner to do.
Prep Artist
They basically do clean up jobs for the live-action footage before the shot can move to the next department. They’re responsible for removing any unwanted items such as markers, wires, or even a scratch. The work of a Prep Artist generally will be reviewed by a VFX Supervisor on a cinema screen.
The images below are a job post from Rodeo FX studio on Glassdoor. They explained what responsibilities they need a Prep Artist to work for them, and also what qualifications you need to have to apply for this job. They explained the Prep Artist is also a Roto Paint Artist.
Roto Artist
They trace the areas of live-action frames using various software to cut images and sequences out to create roto mattes and prepare plates for the compositors. They also work in video game productions which use live-action footage.
The following link explains their responsibilities, skills required, what software will be used, and the average salary
Roto Artist Job Description, Salary, Skills & Software (cgspectrum.com)
I also found a video on YouTube which shows what a Roto Artist’s showreel is like. They generally rendered out the alpha channel to show how clean the plate is.