简单来说：Process.spawn = Process.fork + exec
Process.fork allows you to run ruby code in another process. Process.spawn allows you to run another program in another process. Basically Process.spawn is like using Process.fork and then calling exec in the forked process, except that it gives you more options.
Use fork if you need to run arbitrary ruby code in a separate process (you can't do that with spawn). Use spawn if you need to invoke an application in a subprocess.
Source：
http://stackoverflow.com/questions/4129196/whats-the-difference-between-process-fork-and-process-spawn-in-ruby-1-9-2