I recently saw in an ad that an Italian university invites applications to "Senior researchers (RTD-b, tenure track)." What are RTD-b positions? Since they are said to be tenure track, I guess they correspond to assistant professors in the US system? Are RTD-b position holders expected to do independent research and advise PhD students as assistant professors do? How much teaching load do they usually have? What is their career path after getting tenured?
Exactly, they are tenure-track assistant positions. After 3 years, if you pass a successful evaluation based on your research activity (abilitazione scientifica nazionale), you become an associate professor. The guidelines for this evaluation include an indicative target number of papers and citations, which is field-dependent. This is only indicative, there is a national committee in charge of the evaluation, and they are supposed to always check case-by-case.
The teaching load is quite low: 60-80 hours of frontal teaching per year, plus exams (warning: there is a lot of exams in the Italian system). As far as I know there aren't other obligations, apart from maintaining a good research output to get the abilitazione. Supervising PhD students is not required.
(The teaching load becomes 120 hours after you become an associate professor.)
The system for the final evaluation is still provisional and has to undergo a new reform in the next months. These positions were introduced in 2010, and things are still new and bleeding-edge. But in any case they are worth pursuing, there are not many other chances to get a permanent associate professor position in Italy.
To be eligible, you need at least 3 years of post-doctoral experience (comparable to an Italian "rtd-a" position).
RTD-B is an acronym for Ricercatore a Tempo Determinato tipo B. That is to say, fixed-term researcher of type B. The term is three-years, by the end of it you are supposed to have passed a national habilitation based on a few key metrics, and then become Associate Professor (Professore Associato). Type A on the other hand, is simply a postdoc position.
The "Ricercatore" title is the italian nominal equivalent of a USA Assistant Professor. I specify nominal because, contrarily to USA positions, you are not expected to start a group of your own, your are not given any research money to do so, you are not expected to supervise BS, MS, or PhD students. You do are expected to perform high-class research; the independence of your research will largely depend on the ability to attract funds on your own. In that regard, your moral standing is more like a Research Scientist in USA, unless you are capable of attracting large national grants, or large AND prestigious European grants. With those funds, you will be able to gather all resources you would need for fully independent research.
You will be expected to teach, and the teaching load will vary a bit depending on the institution that hires you. Typically one course per year, that amounts to 50 to 80 hours of formal lectures, to a typically a large number of students, grading exams scattered throughout the year.
One thing to keep in mind is the selection process. In the USA, an institution will receive several applications, make a short list, and invite the shortlisted candidates for one or two full-days interview. In Italy, institutions nowadays use shortlisting as well, but interviews last only 20 minutes or so. Also, recommendation letters are not needed in Italy, although some places are starting to ask them.
After getting tenured, the next path in your career would be to become Professore Ordinario, the equivalent of a USA Full Professor. And then, just as in the USA, you might take other paths as to become Dean.