From: "Hal E. Fulton" hal9000@hypermetrics.comHere's a way to look at it. 
 ruby-talk(51035)
Do you find "each" useful? And other iterators? 

Have you ever wanted to write an iterator for a class
of your own (that didn't inherit from a class already
having one)?

In a case like that, you would use yield.

Have you ever wanted to write (what I call) a "non-iterating
iterator" that does some housekeeping, calls a block, and  
does more housekeeping? Then you would use yield.

Definitive examples of the non-iterating iterator:
  File.open("file") { block }  # open, do block, close
  Mutex.synchronize { block }  # grab, do block, release
  Dir.chdir("dir")  { block }  # cd, do block, cd back

If you don't understand how this works, try this:

def do_something
  puts "Before"
  yield  # Basically call the block
  puts "After"
end

do_something { puts "I'm doing something" }

Output:
  Before
  I'm doing something
  After

Secondly, note that if you want an "iterating" iterator, you
would do a yield inside a loop. Control is traded back and
forth between the iterator and its block.

Thirdly, note that yield returns a value (which is the last
expression evaluated inside the block). This is useful for
things like Enumerable#select and others.