A function is a collection of reusable markup. In this example, there is a subroutine named "foo" that will print "Hello World";

function greeting {
  Write-Host "Hello World"
}

Or like this.

function greeting {
  $hello = "Hello World"
}

In this example, the following markup will invoke the greeting, which will print "Hello World".

greeting

Often, functions have a return.

function greeting {
  return "Hello World"
}

In this example, the string returned in the greeting function is stored in the $hello variable.

$hello = greeting
Write-Host $hello

Often, values are passed into a function.

function greeting($first, $last) {
  return "Hello $first $last"
}

In this example, "Hello John Doe" should be printed.

$hello = greeting 'John' 'Doe'
Write-Host $hello