What is the difference between a job and a process?
A process is any running program with its own address space.
A job is a concept used by the shell - any program you interactively start that doesn't detach (ie, not a daemon) is a job. If you're running an interactive program, you can press CtrlZ to suspend it. Then you can start it back in the foreground (using fg) or in the background (using bg).
While the program is suspended or running in the background, you can start another program - you would then have two jobs running. You can also start a program running in the background by appending an "&" like this: program &. That program would become a background job. To list all the jobs you are running, you can use jobs.
For more information on jobs, see this section of the bash man page.
UNIX has separate concepts "process", "process group", and "session".
Each shell you get at login becomes the leader of its own new session and process group, and sets the controlling process group of the terminal to itself.
The shell creates a process group within the current session for each "job" it launches, and places each process it starts into the appropriate process group. For example, ls | head is a pipeline of two processes, which the shell considers a single job, and will belong to a single, new process group.
A process is a (collection of) thread of execution and other context, such as address space and file descriptor table. A process may start other processes; these new processes will belong to the same process group as the parent unless other action is taken. Each process may also have a "controlling terminal", which starts off the same as its parent.
The shell has the concept of "foreground" jobs and "background" jobs. Foreground jobs are process groups with control of the terminal, and background jobs are process groups without control of the terminal.
Each terminal has a foreground process group. When bringing a job to the foreground, the shell sets it as the terminal's foreground process group; when putting a job to the background, the shell sets the terminal's foreground process group to another process group or itself.
Processes may read from and write to their controlling terminal if they are in the foreground process group. Otherwise they receive SIGTTIN and SIGTTOU signals on attempts to read from and write to the terminal respectively. By default these signals suspend the process, although most shells mask SIGTTOU so that a background job can write to the terminal uninterrupted.