Recently there was some discussion about how to license your mod. This can be a complicated topic, but it doesn't have to be.
Here I will list several common licenses, and what their main features are. Note that I only included those that are best suited to mods
CC0: CC0 is about as close to "public domain" as you can get and still have a proper license. Basically this license says that you don't care what anyone does with your work, and you specifically disclaim all rights to it.
CC BY: Creative commons attribution, this license basically says you can do whatever, but anyone who modifies or uses your work has to credit the author (you!).
CC-BY-SA: Same as the previous one, except if someone modifies or otherwise distributes your work they need to release their changes under the same license you used. This means that if your mod is included in a mod pack, the mod pack need to use a license that is compatible with this one.
Other CC License: There are more Creative Commons licenses, it may be worth checking them out in more detail.
MIT: The MIT license is a very popular open source code license. While it is designed for program source code, it can also be used for other content, such as mods. Basically this license says you can do whatever, so long as the license and copyright notice is included in any distribution.
Zlib: Yet another code license. This one allows you to use the content however you want, so long as you do not claim modified versions are the original, and you do not remove the license from any versions you distribute.
BSD: Like the MIT license, the BSD license is designed for program source code and may be suboptimal for a mod. The common three clause license requires the same basic things as the MIT license, with the addition that the name of the original author (you!) cannot be used to promote works based on your mod without prior permission.
In short, I suggest CC-BY for most mods, CC-BY-SA being pretty good too. If you really could care less, just use CC0. MIT and BSD licenses are great for code, but really are not written for content. Most other licenses are even less suitable in one way or another. In any case, I strongly suggest you follow the link for the license that interests you most and read up on it before using it since I am not a lawyer
It is worth noting that if you harbor hopes that your mod will be integrated into the base game CC0 is probably your best choice. CC-BY-SA on the other hand is a very bad choice in this case.
I strongly recommend reading the next post (by copygirl) too, as that has more info.