The term "grad school" may refer to a Master's degree program, but it also applies to PhD (Doctor of Philosophy), MD (Medical Doctor), JD (Juris Doctor -- the degree typically required to become a lawyer in the USA) or any other degree obtained beyond a Bachelor's degree. Some schools, however, also offer graduate certificates which would most often not be considered true graduate school as they do not result in a Master's or other degree, but rather a certificate indicating specialization in a specific study area.
Grad school degrees may be academic in nature, for example, my degree is a Master of Fine Arts in Writing. Or, a grad school degree may be professional in nature, such as an MD (Medical Doctor) or NP (Nurse Practitioner) in which the degree is specific to a professional field and typically results in specific licenses and certifications beyond the degree.
A "terminal" graduate (grad) degree is the highest degree available in one's area of study.
Master's degrees typically take one to two years of full time study after an undergraduate degree is earned. Other graduate degrees may take anywhere from three to seven years (or more). Some universities have accelerated programs where graduate credits may be earned in conjunction with undergraduate study in order to accelerate the timeline to degree achievement.
References:
https://career.berkeley.edu/Grad/GradWhatis
https://www.bankrate.com/finance/jobs-careers/masters-degree-or-graduate-certificate.aspx
https://guides.lib.uw.edu/bothell/gradschool/gradprof
