Difference of asynchronous and synchronous programming

gistfile1.js

Asynchronous programming
JavaScript:
var a = 'foo';
//lots of code
setTimeout(function(){  //Beginning of code that should run AFTER the timeout
    alert(a);
    //lots more code
},5000);

alert('before foo');

Synchronous programming
Ruby:
def foo
    sleep(5)
    yield
end

foo { puts 'foo'}
puts 'before foo'