The Bash & (ampersand) is a builtin control operator used to fork processes. From the Bash man page, "If a command is terminated by the control operator &, the shell executes the command in the background in a subshell".
If logged into an interactive shell, the process is assigned a job number and the child PID is displayed. The job number below is one.
bash$ sleep 30 &
Note that when a process is forked, the child PID is stored in the special variable $!
bash$ echo $!
You can terminate the job by its job number like so:
+ Running sleep 30 &
bash$ kill %1
+ Terminated sleep 30