Difference between 2 tier, 3 tier and 4 tier?

2 Tier the user connects directly to the Database, in 3 Tier the connection to the database goes through the application server. For 4 Tier the connection established from Web Server and Application Server to Database.

Examples for 2 Tier - DataMover / Application Designer connection with Connection type oracle, SQL Developer connection, sql connection with command prompt etc.,

Examples for 3 Tier - Application Designer/DataMover with connection type 'application server',

Examples for 4 Tier - PIA connection, PTF etc.,