Are the details you share with a recruitment agency kept confidential?

I was recently hired at my current company through a third-party recruitment agency. The recruiter who placed me in my role has been checking in on me periodically and has suggested we meet up soon. I’m curious about whether the information I share with recruiters remains confidential and won’t be passed on to my employer or colleagues.

I recently discovered that my salary is significantly lower than that of my colleagues, which was quite disappointing to learn considering I take on a heavier workload. I’m wondering if it’s appropriate to discuss this kind of information with the recruiter.