There are two primary ways to prompt the user for input. You can either use confirm to display a multiple choice dialog (such as yes/no/cancel), or input to process a more complex input.

The confirm function prompts the user with a dialog and a multiple answers a user can select from. Let's try a simple example:

let answer = confirm('Is cat your favorite animal?', "&yes
&no")
echo answer

If you execute the script, you'll get the following prompt:

Hitting y or n will select an option. Let's hit y:

The result of this is 1. Now, what if we replay it and choose no?

We get a 2. As you can see, confirm returns an integer with the number of the selected choice.

Oh, and if you're running from a GUI, you'll get a dialog window pop up:

Now, let's get back to our original example:

let answer = confirm('Is cat your favorite animal?', "&yes
&no")
echo answer

Here, you can see that confirm takes two arguments: a prompt to be displayed, and a newline-separated ( ) list of options to select from. In the previous example, an option string is non-literal, since we want the newlines to be processed.

A set of ampersand & symbols are used to denote the letters representing each option (in the previous example, y and n become the available options). Here's another example:

let answer = confirm(
   'Is cat your favorite animal?', "absolutely &yes
hell &no")

This would display the following prompt:

Note that y and n are still the letters a user can press to reply to the prompt.

Lastly, input lets you work with free-form text input. Its use is fairly straightforward:

let animal = input('What is your favorite animal? ')
echo "
"
echo 'What a coincidence! My favorite animal is a ' . animal . ' too!'

And this is how the prompt looks when executed:

And this is what it looks like after we enter our string:

However, a word of warning. If you're using input from inside a mapping, you must prefix it with inputsave() and follow it with inputrestore(). Otherwise, the rest of the characters in a mapping will be consumed into input. In fact, you should always use inputsave() and inputrestore() in case your function is ever used in a mapping. Here's an example of how to use them:

function AskAnimalName()
  call inputsave()
  let name = input('What is the animal"'s name? ')
  call inputrestore()
  return name
endfunction

nnoremap <leader>a = :let name = AskAnimalName()<cr>:echo name<cr>