Medical statistics review (assuming some basic familiarity):
Mean (average), median (middle number), mode (most common number)
Sensitivity and specificity depend on a specific, pre-determined test. Also, they do NOT depend on prevalence of the condition being tested for.
Positive predictive value and negative predictive value depend on the prevalence of the condition being tested for.
Odds ratio: People who have A,B,C are x times more or less likely to have a certain condition/outcome.
Relative risk: People who have A,B,C are x times more or less at risk to have a certain condition/outcome.
The key difference is that relative risk depends on the prevalence of the condition/outcome. In the situation that the condition/outcome is rare, odds ratio and relative risk approach equivalence.