This is a collection of reader macros (think syntactic sugar) that are valid in Clojure. These macros are useful for commenting out expressions, defining sets, ...
All reader macros start with the character #, which is infact the Dispatch macro that tells the Clojure reader (the thing that takes a file of Clojure text and parses it for consumption in the compiler) to go and look at another read table for the definition of the next character - in essence this allows extending default reader behaviour.
#_ - Discard macro - ignore the next expression. Often used to comment out code, especially when nested inside other expressions
#' - Var macro - returns the reference to the var. Used to pass the definition of something rather than the result of evaluating it.
Reader macros are part of the Clojure language specification, so are different to macros, which can be defined by anyone.