When it comes to releasing your software publicly, you need to specify at what conditions may other people use, modify, copy and redistribute it. This is usually done by supplying some kind of a document along with your software called license. Writing a good license could mean having a pretty rough times over a fat pile of lawyer books for a common software developer though.
Luckily for us, there are many licenses already available, that you can use and under which you can distribute your software. Some are more restrictive than others and it's only up to you which licensing path you want to take. Since you're reading my blog about Linux, I'll assume you probably want to distribute your application's sources as well. There are numerous ways to go. So let's take a look.
One way to publish your software is by forfeiting your copyright a thereby releasing it into public domain. Anything that belongs to the public domain is without copyright. That means, that everybody can do anything they want with is. This might be useful for someone.
The thing is, that by default, if you write a piece of software, you automatically become it's copyright holder. This is stated in a thing called Berne Convention, which most of the countries have signed. So, in order to make your software public domain, you need to make clear, that you forfeit your copyright.
Open Source vs. Free Software
Another great ways to distribute your software. Generally, there are two ways. Using an open source license or a free software license. What is the difference between these two? Well, there are two separate camps – OSI (Open Source Initiative) and FSF (Free Software Foundation).
By making your project open source you allow others to "do whatever they want" with the sources (as long as you don't violate the licensing terms). The open source is more practical. It gives the user a lot of freedom in copying and modifying the software.
On the other hand, making your project a free software will allow users to "do whatever they want" (under the terms of the license) and if they modify the software, they have to publish the changes and give it back to the community. This makes the derivative works also publicly available. The free software is more philosophical. It wants to make sure, that the software once released as free will retain it's freedoms.
Here are some examples of popular and widely used licenses for your software. My personal favorite is GNU GPLv3.0. It's a free software license, pretty restrictive about usage in proprietary software, but I like the philosophy behind it. It was initially written by this guy named Richard Stallman. The lesser version of this license (allowing linking in proprietary software) is GNU LGPL.
Other popular ones are the BSD licenses. There are multiple variants of the BSD license. They are more open source, than free software.
The MIT License is pretty straight-forward as well as permissive.
Great comparison of various free software and open source licenses is available on Wikipedia.