You see it more and more often, a server that runs on Spigot, but what exactly is it?
Bukkit was a popular server version in which you can use plugins. Spigot was another extension of this, making everything just a little faster in the background. You see no difference in game. Bukkit stopped at a time while Spigot is still being developed.
Spigot for vanilla Minecraft
Spigot is also very suitable for vanilla Minecraft, as long as you have no plugins in the server you will see no difference. The reason to use Spigot is because Spigot is a lot faster than the standard Minecraft server jar.
If you want to use plugins, Spigot is the ideal jar for your server, there are more and more alternatives to Spigot, but these are not yet used on such a large scale, so with Spigot you are sure that you can achieve good performance on your server.