Open Source software as the name suggests is a software which is free to use by everyone. Here the source code of the software is released under a license in which the copyright holder grants users of his software the rights to use, study, change and distribute the software to anyone and for any purpose. An Open Source software is made under an Open Source development model which is the process used by an Open Source community. The software is then released under an Open Source license so that anyone can view or modify the source code. Again community powered approach means one can find help, resources, and perspectives that reach beyond one interest group or one company. The beauty of an Open Source software is flexibility wherein due to it's emphasis on modification, one can use Open Source code to address problems that are unique to your business or community.
Middleware in a layman language can be called as a " Software Glue". Middleware is used to connect your legacy apps to your modern cloud apps, your cloud apps to each other, and anything to everything else.
One may think it of a Software layer that lies in between the Operating System and Applications in a distributed computing environment. Thus Middleware is a computer software that connects various software components or applications.It consists of web servers, application servers, content management systems, and similar tools that support application development and delivery.
DevOps as the name stands for is Collaboration between Development and Operations Team. It is a set of practices to be followed which shortens the System Development Life Cycle (SDLC) and provides continuous delivery with high quality software. DevOps accelerates the process to deliver applications and software services at high speed and high velocity.