In C there are some differences however:
Purpose is to create a new process, which becomes the child process of the caller
Both processes will execute the next instruction following the fork() system call
Two identical copies of the computer’s address space,code, and stack are created one for parent and child.
Thinking of the fork as it was a person; Forking causes a clone of your program (process), that is running the code it copied.
Purpose is to create a new thread in the program which is given the same process of the caller
Threads within the same process can communicate using shared memory. (Be careful!)
The second thread will share data,open files, signal handlers and signal dispositions, current working directory, user and group ID’s. The new thread will get its own stack, thread ID, and registers though.
Continuing the analogy; your program (process) grows a second arm when it creates a new thread, connected to the same brain.