A distro is the server players use to host their server. The most common distros are Evolutions and The Forgotten Server (also known as TFS). A small description of each server can be found below. Distros may include many features, but some don't have anything special in it at all. Also most distros can be modified as the hoster wants it. A person who edits the distro is name a coder.
Evolutions: Evolutions is probably the most common xml server. It's mostly used by beginners or somewhat lazy people. Although this server has some drawbacks like: It takes long to edit, when you switch map, you need to set every temple spanw manually. It goes laggy at a high amount of players. You're not able to use an account site.
The Forgotten Server This is the most used sql server, which means it has a database where it saves all player, house and account files (not the house.xml file from the map). This server is used a lot since it is easy to edit (by 1 easy code line you can change all the temple and spawn position of a player), as long as your pc and internet are good, you can handle large amounts of players and you're able to use an account site.