Two-tier Architecture: In the two-tier architecture, the application is partitioned into a component that resides at the client machine and calls the database system functionality at the server machine using query language statements. Application program interface standards like ODBC and JDBC are used for interaction between the client and the server.
Three-tier Architecture: In three-tier architecture, the client machine contains a form interface and does not contain any direct database calls. It communicates with an application server which in turn communicates with the database system. Thus, the required functionalities are not distributed among the multiple clients. This architecture is appropriate for large applications that run on the World Wide Web.