How to communicate between two .exe ?

Hi All,
i am creating .exe of my two diffrent wpf applications which will perform upload/download(backup). first app is storing information from user for backup and store in database and also displaying status of previously performed task in datagrid. the second app is working as a background service which will read the database and will perform the upload /download operations.
now what i want is that whenever my second app complete any operation it should send msg to my first app to notify that operation is completed so that my first app update its GUI deatils regarding operations.
so how can i send msg through one .exe to another .exe
Please tell me the best possible way to do it.
thanks