What's the difference between "Retail" and "Nulled"

Retail = Original version without any modifications, directly received from the creator.
Nulled = Removed licensing or bypass licensing, sometimes code modifications, removed callbacks ect

It always depends on the script you're using. If you're just using a HTML template, it doesn't matter if you're using a retail version, because they don't have callbacks. For scripts, it's better to use a nulled version to be safe. (As long as your hosting allows it of course.) Like simdol8080 said, be sure to download nulled versions from places you trust.