Typically this means the exam that comes after a coordinated block of classes, such as in preclinical years in a systems-based curriculum. So if you do a neuro block, your neuro block exam would cover physio/anatomy/biochem/etc topics for neuro in one exam, instead of a bunch of separate exams for the neuro topics within physio/anatomy/biochem/etc.
By contrast, in a traditional curriculum where you take biochem all by itself, physio/histo/anatomy all by themselves etc., you aren't doing blocks. So your biochem exam covers everything your school is going to teach you in biochem across all systems.
Typically this means a "stock" exam, such as those published by NBME, which tests you on a single subject where the material is well-established and standardized. Your school might use the biochem shelf, for example, but I think this is fairly rare.
Most likely a "shelf exam" means an exam given by your school during 3rd/4th year rotations, such as the IM shelf, the psych shelf, etc.
These are the classes you take to prepare/emphasize clinical subjects such as patient exam, taking vitals, etc. "Clinical" usually refers to what you do when there's a patient involved. Non-clinical classes are things like biochem, physio etc. Jurisprudence and ethics are non-science classes, usually taught in pre-clinical years, that don't fall neatly into clinical or non-clinical.
Which brings us to the term "didactic" which means anything that is taught by an instructor. If your school does not have didactics during 3rd/4th year, thus leaving you on your own to learn what's on the boards, that's not good.
Google, while always friendly, in this case is indeed useless.
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