setinfo

setinfo
is a console command available in all Source games.
setinfo
is a console command that creates a new console variable. The variable and its value will be preserved across map loads, and (possibly to your downfall) game saves. There doesn't seem to be a way to mark one as a cheat.
Syntax
setinfo VAR VALUE
If you wanted to make a variable to count how many NPCs you kill, you could type setinfo mykills 0
. (Or you can set it to any impressive number ;) )
mykills
would become a variable just like anything else, such as sv_pure
.

Convar VAR is already registered but not as user info value
If you see this in the console, it means you tried to set a console variable that already exists, such as sv_cheats
.
See Also
incrementvar
, a convenient command for counting things.