A part of Concurrency concepts.
Under Unix at least, processes are the data construct used by the heart of the operating system when invoking a particular program.
In Tcl, exec and open |... are your friends for starting helper processes.
To obtain a ps(1)-like list of Windows processes, use pslist from pstools. [tlist?]
To obtain a ps(1)-like list of Solaris or Linux processes, use procfs.tcl from http://inferno.slug.org/wiki/TclPsLib