Router#

Set a hostname:

 Router(config)#hostname r0
 r0(config)#exit
 r0#

Note that both the main prompt, and the config prompt say "r0"

Set the prompt with "prompt"

 r0#conf t
 Enter configuration commands, one per line.  End with CNTL/Z.
 r0(config)#prompt Your_Wish#
 r0(config)#exit
 Your_Wish#

"Your Wish#" overrides the main prompt, not the configure prompt.
sean@ertw.com

%h - Host name. This is either Router or the name defined with the hostname global configuration command.
%n - Physical terminal line (tty) number of the EXEC user.
%p - Prompt character itself. It is either an angle bracket (>) for user EXEC mode or a pound sign (#) for privileged EXEC mode.
%s - Space.
%t - Tab.
%% - Percent sign (%)
sean@ertw.com