As far as I know, PostgreSQL is simply a relational database, not a GIS. PostGIS is the project built on PostgreSQL that makes it a spatial database that can be used by GIS systems. It's whole purpose is to store the information and to serve it up when requests are made by a client. There is no 'server screen' that needs to react; not in the sense of a GUI on the server side, at least.
I can say, not knowing much about PostgreSQL, that I'd look for documentation, examples or a book on how to interact with the database using a web client. There are probably user groups for this stuff, too, if you just google it. You should be able to use any technology, but I would guess PHP is going to be a good solution for you.
Daniel
So may design questions ......
Programming
What languages are you restrained to ? JAVA, ASP.NET, etc. ?
Are you using the Google Maps API to retreive the lat/longs of the point on the map ?
Database
What are you storing on the database ?
The lat long values of the locations ?
Maybe a table with lat/long, name of location and maybe a brief description ?
How are you getting these locations into the database ? Is that part of your interface ?
How are you restricting access to the database? Do users need to sign on somehow ?
Back to your questions .
Typically the web interface sends off a query to the server and retrieves a message back. The return message is then interpeted and acted upon. One of the more popular ways to do this is thru SOAP and XML. On the server side you may need a helper app to gather the requests, interface with teh database, and send back the messages.
Others may have different opinions.
HTH
Jerry