How to remotely set the focus on a window

[Free Ekanayaka]

is there a command line utility or C/Python library, to tell Xorg
to  set the focus on a  certain window?
And moreover how can get the list of  the opened windows? Do they have
an ID or a name that can be passed to the such command-line-tool/library?
What I want to  achive is to be able  to write a graphical application
that simply select other windows and bring  them on top and with focus
set (pretty much like what  the window selector   of a window  manager
does).
Ciao,
Free