What Is My License
There are licenses and user agreements everywhere while we are used to thinking about this when programming it also holds up for our written works, podcasts we produce, and if you are of the musical bent: songs you write or cover. So let's dive into some of the more popular licenses out there and what they mean for you.
MIT
The MIT license is pretty much the most flexible license out there. With an MIT license you can do anything you want with the provided IP. You could take just a piece, you could use the whole thing. You can create a new open source project, or you could use it to power your evil Moon base.
There are two catches. You have to accredit the work that you are using somewhere and say that it uses the MIT license (usually there's a license file somewhere). Second, you can not take any action if the work fails you somewhere. So if that library you are trusting with your massive enterprise solution has a breaking bug that costs you millions in business: you're out of luck legally.
Apache
The Apache is pretty flexible, but there's a few more restrictions than the MIT. Essentially, you have to keep the license files and attribution just like the MIT. On top of this, if you modify the source work, you have to make a note of these modifications. You can distribute and license your work anyway you want on top of Apache including the modifications you have made.
Just like the MIT license, the Apache license gives no warranty.
GPL
GPL is really tricky (especially when you consider that the GPL is licensed for use by the GPL [Stallman likes his recursion]). Things only get more complex since there are multiple versions of the GPL and many iterations allow for custom clauses while maintaining the GPL label. If you use GPL code as part of your product, then you will have to license your source as GPL and make it available. However you can use GPL in your services so don't be afraid of installing Ubuntu on your next web server.
GPL is one of the longer and more complex licenses on this list. If you find yourself creating a product heavily based on an existing GPL library, I would suggest consulting a legal council and thoroughly reading the licensing.
EULA and Special Licenses & Agreements
While very common in our every day lives, you have to read each license a bit more carefully. EULAs and custom licenses can and often do limit what you can do with a product, source code, or other work.
For instance the EULA for Amazon's EC2 service does not allow you to create mail servers using the service without permission of Amazon AWS. This can limit competition, mitigate risks, or limit liability.
No License Listed & General Copyright Law
If no license or agreement is listed then a work falls under general or open copyright law. All rights belong to the original creator. You can not use any part of the work without explicit permission from the original creator and a type of contract (verbal, digital, etc) must be created to allow use of the original work in your derivative work.
A big thing to note is that it does not matter how I distribute the content. This blog for instance is free to view to the public. You can link to it to your heart's contempt. However, if you take a phrase, sentence, paragraph, etc. and use it without my permission, you are infringing on my copyright (even though none is listed)!
By most, it is thought that all that matters is attribution. If I copy an article from a site and put it on mine, but link back to the original, list the author, etc, that is OK because I'm clearly not trying to pass this off as my own. However, this is infringement! You have revoked that author's right to limit distribution of that work. If I decided tomorrow to put all of these posts behind a pay wall, and someone had copied them on their own site, I no longer have the power to restrict the distribution of my content to my site.
A word on payed content. Purchasing a work does not grant you license to use its contents in a derivative work. If you were to purchase my book for instance, that does not give you the right to take chapter 2 and publish it on your site, use it in a book of your own, or use it for any derivative work. Just like anything else you must receive and prove redistribution rights.
Non Derivative Works and Quotes
This is where things get tricky. For instance, I currently have someone writing a review of my book: they are able to without my consent quote small fragments (sentence or so) of my book within their review. The trick here is that this is not considered a derivative work. This falls under the category of fair use.
In general fair use contains: academic works, reviews, and parody. However, fair use is seriously tricky. It really is up to the original copyright holder to decide to seek action. If you are using something in what you see as fair use, it is best to check with the original author and attribute where ever possible.