Open source software is defined as the software that available with the source code. All commercial software iswritten by the developers with source code. This code is the language to instruct computer to operate as defined. Therefore, the code is not easily for layman to read and understand. After the translation from the compilers, computer can run the program as usual. However, people could not see the source code again. Which Means Open Source is such software without source code accompanying with.
Most of the commercial software, such as the Microsoft Office is not open sourced. Therefore, you can only use the developed functions. For instance, after you purchased the Microsoft Office, you could only use the functions offered in the software. You could not design the special functions for you, and also change the user platform in order to be convenient to you.
On the contrary, the open source software is downloaded with the source code. If you have knowledge on coding, you can update the source code by yourself. Which Means Open Source is to encourage the users to design and modify the source code. After modification, the software shall be more useful, addressing the needs to end customers. In addition, some commercial software is just released in the world for download; however, they might have bugs. The developer only corrects the bugs by receiving feedback by customers. However, the open source software is distributed with the source code; therefore, you can update the software when you find bugs.
Most of the open source software is free of charge. As mentioned above, the goal of the software is to attract correction from customers. Therefore, free of charge can attract people to download and test.
The only drawback of the open source software is that there is no guarantee on the warranty. Since you download the software free of charge, the developer does not have the responsibility to update the software regularly.