Everything in Unix is either a file or a process.
When you run a program, a process is created. Every process is identified by a number called process ID. To check the processes you are running, execute "ps" command on the shell. You can think of the process ID to be a sequence number given by the operating system. It may be different at different execution of the same program.
A file is a sequence of data. A file could be created by users using word processors or text editors or by the program to keep the information. A program is kept in the form of a file and when it is run by the kernel, it loads as a process.
A file is generally written on the disk so that it exists even after the computer restarts. It is saved in a disk - either hard disk drive (HDD - cheaper and slower) or solid state drive (SSD - faster but costlier).
A file is identified by a name called file path. In Unix, everything is represented as file:
Devices such as Mouse, Keyboard
Programs are saved as file
Disk and Monitor