What are the differences between fork and exec?
The use of fork and exec exemplifies the spirit of UNIX in that it provides a very simple way to start new tasks. Note the use of the word task here, I have deliberately avoided using the terms process or program, which you can define as:
Note that you can have the same program (such as an interactive shell) running in different processes.
With that in mind, the fork call basically makes a duplicate of the current process, identical in almost every way. Not everything is copied over (for example, resource limits in some implementations) but the idea is to create as close a copy as possible.
The new process (child) gets a different process ID (PID) and has the PID of the old process (parent) as its parent PID (PPID). Because the two processes are now running exactly the same program, they tell which is which by the return code of fork - the child gets 0 and the parent gets the PID of the child. This is all, of course, assuming the fork call works - if not, no child is created, and the parent gets an error code.
The exec call is a way to basically replace the entire current program in a process with a new program. It loads the program into the current process space and runs it from the entry point.
So, fork and exec are often used in sequence to get a new program running as a child of a current process. Shells typically do this whenever you try to run a program like find - the shell forks, then the child loads the find program into memory, setting up all command line arguments, standard I/O and so forth.
But they're not required to be used together. It's perfectly acceptable for a program to fork itself without execing if, for example, the program contains both parent and child code (you need to be careful what you do, each implementation may have restrictions). This was used quite a lot (and still is) for daemons which simply listen on a TCP port and fork a copy of themselves to process a specific request while the parent goes back to listening.
Similarly, programs that know they're finished and just want to run another program don't need to fork, exec and then wait for the child. They can just load the child directly into their process space.
Some UNIX implementations have an optimized fork which uses what they call copy-on-write. This is a trick to delay the copying of the process space in fork until the program attempts to change something in that space. This is useful for those programs using only fork and not exec in that they don't have to copy an entire process space.
If the exec is called following fork, that causes a write to the process space, and it is then copied for the child process.
Note that there is a whole family of exec calls (execl, execle, execve and so on) but exec in context here means any of them.
The following diagram illustrates the typical fork/exec operation where the bash shell is used to list a directory with the ls command:
+--------+
| pid=7 |
| ppid=4 |
| bash |
+--------+
|
| calls fork
V
+--------+ +--------+
| pid=7 | forks | pid=22 |
| ppid=4 | ----------> | ppid=7 |
| bash | | bash |
+--------+ +--------+
| |
| waits for pid 22 | calls exec to run ls
| to finish V
| +--------+
| | pid=22 |
| | ppid=7 |
| | ls |
V +--------+
+--------+ |
| pid=7 | | exits
| ppid=4 | <---------------+
| bash |
+--------+
|
| continues
V
As an aside, there's an interesting answer over on the Retrocomputing Stack Exchange site that details some history of fork and exec.