OSDev is the hobby and process of developing a custom operating system and/or kernel. Building a Linux distro is typically not considered OSDev.
Traditionally, low-level systems programming languages (such as C and C++) are used for Operating System development, but theoretically any language can be used after porting a runtime for it.
When these operating systems are not written or used in a professional context, OSDev can be considered Recreational programming.
There are various online communities around hobbyist operating system development. The most notable one is probably osdev.org.