Recs.
Updated
a Unix-like and mostly POSIX-compliant computer operating system assembled under the model of free and open-source software development and distribution
SpecsUpdate
Pros
Pro Web servers are built for it
Unless you have Windows network environment and technologies, installing most web technologies on Linux is usually pretty easy. Even there are many distributions, there are less package managers than distributions and installation is nearly similar on all *nix.
Pro Small attack surface area
Linux servers have frequently very few ports open by default, few services behind any port, and a general attitude toward having each service run with very limited privileges. Most services running on Linux are build with that in mind. The security model for is mostly around user and groups, so it's not as powerful than Windows permission system, but simple is often good to avoid holes.
Pro Easy to setup development environment
It's very easy to setup the development environment. In fact most of the time you don't even need to do much. For an example you don't even have to install GCC if you are coding in C/C++. Some distribution comes with JDK or JRE pre installed. Even if it doesn't, it's very easy to install a JDK than on Windows. There are plenty of free and open source / proprietary IDE's available.
Pro Familiarity with Linux is often required from a developer
Many university computer science programs are based on Linux and in any case, you will inevitably be dealing with a Linux box of one flavor or another someday, be it a server (most likely) or a workstation. The languages and methods used in the Linux/Unix environment (e.g., bash, C, C++, Make, etc.) are very commonplace among developers and are to the computer side of the discipline what the English language is to the human side of it: the common language.