A three tier client-server architecture is common for web applications, it consists of the client, the application / web server and the database server.
The intermediate layer (application / web server) stores the web connectivity software and the business logic part of the application used to access the corresponding data from the database server. Acts like a conduit for sending partially processed data between the database server and the client.
This can enhance security, as the database server is only accessible via the middle tier and clients cannot directly access it.