Whats the difference between CraftBukkit and Spigot?

Whats the difference? I'm trying to decide over one. In the past some of my plugins didn't work with spigot and you couldn't place some items. So today, whats the difference and which one should I choose and why.

 

Spigot is a fork of CraftBukkit (-> a modified version)

The main difference is the different philosophy. While Bukkit tried to change as little as possible to vanilla code and as safe as possible, while Spigot is willing to take more risks.

For example does Spigot use protocol hacks to allow clients of different, incompatible, versions to play together on a server.

However, bigger changes means bigger room for bugs, especially if you do things that are not meant to be.
E.g. Spigot got a critical bug recently, that caused every server to crash thanks to a bug.

Also you should know that you won't get any support for Spigot in this forum, as it falls under the "unofficial build policy".
So if you don't know for what you need Spigot in favor of CraftBukkit you should stick with CraftBukkit. It's in general more stable.

 

Syd
Well, I don't want to offend someone, but you can't really run big server with CraftBukkit only. It will start lagging a lot after certain amount of people. All really big servers uses Spigot or another forks to allow more people. Some things in default minecraft are very inefficient, and requires changing to prevent lags. Running Spigot can cause some problems with plugins and will be more difficult then just Bukkit, but when you need to choice between "vanilla-like with minimum risks, but laggy" and "optimized, medium risky, but could handle much more people" I think you'll not think too much.

 

rsod said performance is achieved with disabling vanilla functions, which is possible using bukkit configuration and API.

Also its not an issue to run multiple servers, if one server is full, players use the other server IP.