На примере 2-х хешей
hash={:fruit=>"Apple", :vegetable="Cucumber"}  и   hash={"fruit"=>"Apple", "vegetable"="Cucumber"}.
Расскажите в чем разница между этими сущностями и где применяют symbol.

What is symbol GC and why should you care? Ruby 2.2 was just released and, in addition to incremental GC, one of the other big features is Symbol GC. If you've been around the Ruby landscape, you've heard the term "symbol DoS". A symbol denial of service attack occurs when a system creates so many symbols that it runs out of memory. This is because, prior to Ruby 2.2, symbols lived forever. For example in Ruby 2.1:

data structure used by a language translator such as a compiler or Interpreter