What decides the seemingly random generated URL parameters?

Question

I know that most sites have different sections and that the url parameters decides on which section to show the user. But if we take pastebin.com for example and create a paste with some data, we can later return to that data with an url that would look something like pastebin.com/mfyNkdFY

How is the value after the .com/ decided? Is it just randomly generated then checked to see if that random string already exists?

And if not, would it be any way to predict what string could be next?