An operating system is a program that acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs. Operating systems have kernels which are responsible for scheduling, starting and ending programs but also provide other functionalities like networking or file systems.
The POSIX standard also defines functions/system calls for file I/O which are commonly found and used on unix systems. You have to remember that in unix everything is a file so these system calls aren't just used for text files but also multiple other things like devices, sockets etc.
POSIX Message Queues
Threads are similar to process and allow a program to do multiple things at once by having multiple threads in it. A key difference between threads and process is however that threads share the same global memory and just have their private stack for local variables and function calls and are therefore not as expensive as process which have the big overhead of creating an entire new memory space. This is why threads are also often called lightweight processes.