Licensing

Software is copyrightable, but the source code can be made available to users however the author chooses. A copyright license is a legal document that tells people how the software can be used and what limitations come with using it.

The more successful emulation projects are often open source (though you definitely will find exceptions).

Public domain

There is no copyright (i.e. No Rights Reserved). Works enter the public domain when they:

1. were released before the current copyright expiry date. This is why old paintings, plays, and books are so commonly quoted and used in modern works, because they'd have to negotiate the rights with the author otherwise. Most software is not released this way because it is still covered by the current American copyright term.
2. are dedicated through a license like Creative Commons Zero or the Unlicense. This is the only option for modern works to be released into the public domain because, per the Berne Convention, copyright is seen as opt-out, not opt-in. If a public domain dedication can't be made (probably because the jurisdiction doesn't recognize the public domain), then the license grants users the equivalent freedoms.

Open-source

The program is released under a copyright license that permits four freedoms: that it can be run at any time, studied and modified for the user's own purposes, distributed to anyone, and improved for everyone else. This bypasses most of the issues encountered with public domain works. For anything else copyrightable, the term "open content" often applies.

Common open-source licenses:

• GNU GPL - This is a copyleft license, everybody has the freedom to do whatever they like with the software (even selling it), the only caveat is: if you make changes, you have to publish the modified source code upon request.
• MIT/BSD/Apache 2.0 - These three are basically the same and similar to Public Domain, you have complete freedom and can do whatever you like with the software. The difference between this and GPL is that these are not copyleft: you can fork a project and turn it closed-source, you don't have to publish the source code changes.

It's worth noting that open-source does not replace copyright. And likewise, the license cannot be removed after the work has been released under it. To see the various open-source licenses available, see choosealicense.com. Also see the appendix at the same website.

Source-available

The program is released under a copyright license more restrictive than an open-source license, but the source code is still publically available. The biggest example is Snes9x, which is released under a non-commercial license. This license makes it not open-source, as it restricts the users' commercial use.

Closed-source / Proprietary

The program's source code isn't available. Often because the ecosystem behind the platform is closed, sometimes by nature (like Windows and Android), or sometimes by force (like every modern console).

Freeware

The source code isn't available but the program is still free.

Shareware / Trialware

A limited demo version of the program is free. This was common for DOS games.