What's the difference between Spigot and CraftBukkit Minecraft servers?

Spigot is a fork of CraftBukkit and implements the same Bukkit plugin API, and is designed to be a drop-in replacement for CraftBukkit.

The advantage of Spigot is extreme configurability. If you know what you're doing, you can see a 50% drop in RAM and CPU use. But that's a big if – since you're asking, you should probably assume that the extreme configurability of Spigot will cause you more trouble than it's worth right now. Spigot is kind of like the "Enterprise Server" version of CraftBukkit, intended for expertly-tuned dedicated servers.

Spigot does update slower too, since it depends on CraftBukkit development.

On the plus side, since they're compatible you can always start with CrafBukkit and then move to Spigot once you feel like you understand how to manage a Bukkit-based server. Believe me, there is plenty to learn just in setting up a "basic" CraftBukkit server.