Difference between "do end" and "{ }" in Ruby

"{ }" has a stronger connection than "do end".

def foobar a, b
 if block_given?
   yield a, b
 else
   puts a * b
 end
end

foobar is called with block.

foobar 2, 3 do |a, b|
 puts a + b #=> 5
end

b is called with block.

def b
 yield
end

foobar 2, b { 3 } #=> 6