collect {|item|block} and map{|item|block} do the same thing – return an array of things returned by the block.  This is different from returning specific items in the collection being iterated over.

Which leads to select.

select{|item|block} will return actual collection items being iterated over if, for each item, the block condition evaluates to true. Not the same as returning what the block, itself, may return.  In the case of select, the block would always return an instance of class TrueClass or FalseClass.  Typically, [true, false, ..., true] is not what you’re looking for in your resulting array.

Slightly modifying the core RDoc example:

a = ["a", "b", "c", "d"]      #=> ["a", "b", "c", "d"]
a.map {|item|"a" == item}     #=> [true, false, false, false]
a.select {|item|"a" == item}  #=> ["a"]