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Q: What is an IDN? Do I need this plugin?

A: If you don't know, you don't need it.

In a nutshell: IDN is an abbrevation for "Internationalized Domain Name"

Internationalizing Domain Names in Applications (IDNA) is a mechanism defined in 2003 for handling internationalised domain names containing non-ASCII characters.

These names are typically written in languages or scripts which do not use the Latin alphabet: Arabic, Hangul, Hiragana and Kanji for instance.

Read more, IDN info at Wikipedia

Q: Do you use a IDN library in this plugin?

A: Yes, this one, Php_Net_idna. Many thanks to the author for their fine work on this great library!

Q: My URL is an IDN in the .com space, how come Firefox show's http://xn- then some characters that are not my IDN?

That is the Punycode representation of your IDN. There is an new ('experimental') Firefox add-on, IDN Navbar, which will display the IDN instead of Punycode. So, if using Firefox and this plugin you (and your visitors) may want to install that addon.