Rspec describe method syntax

Rspec describe method syntax References I don\'t find syntax of Rspec describe method, but find some examples. If I correctly understand, we can pass into describe method a string, a class name (model name, for example), and a string and class name together as a parameters. What difference between these three cases of invocation describe? describe \'string\' do ... end describe ModelName do ... end describe \'string\', ModelName do ... end