A .cs mod is a special GTA:SA modification called CLEO.
To use this mod, go to http://cleo.sannybuilder.com/ and download CLEO 3 Library. This will install many components into GTA:SA and installation is easy and simple.
To install a CLEO mod, simply put the .cs file in the folder named "CLEO" located in your GTA San Andreas directory. Some CLEO mods may be different, so remember to always read the readme file if there is one.
There are many CLEO mods including call for backup, call for a taxi, Speedometers, gas, and etc. Be aware that some of these mods don't work sometimes, and may crash GTA:SA (at least for me). But an upside is that you don't have to start a new game when new scripts are installed.
Thanks for reading, and enjoy!