JDBC types and drivers:

Type 1: JDBC-ODBC Bridge
The type 1 driver JDBC-ODBC Bridge translates all JDBC calls into ODBC (Open
DataBase Connectivity) calls and sends them to the ODBC driver. As such the
ODBC driver as well as in many cases the client database code must be
present on the client machine.
Type 2: Native-API/partly Java driver
JDBC driver type 2 -- the native-API/partly Java driver -- converts JDBC
calls into database-specific calls for databases such as SQL Server Informix
Oracle or Sybase. The type 2 driver communicates directly with the database
server; therefore it requires that some binary code be present on the client
machine.
Type 3: Net-protocol/all-Java driver
JDBC driver type 3 -- the net-protocol/all-Java driver -- follows a
three-tiered approach whereby the JDBC database requests are passed through the
network to the middle-tier server. The middle-tier server then translates the
request (directly or indirectly) to the database-specific native-connectivity
interface to further the request to the database server. If the middle-tier
server is written in Java it can use a type 1 or type 2 JDBC driver to do this.
Type 4 Pure Java Driver
Type 4 JDBC drivers are direct-to-database pure Java drivers ("thin"
drivers). A Type 4 driver takes JDBC calls and translates them into the network
protocol (proprietary protocol) used directly by the DBMS. Thus client machines
or application servers can make direct calls to the DBMS server. Each DBMS
requires its own Type 4 driver; therefore there are more drivers to manage in a
heterogeneous computing environment but this is outweighed by the fact that
Type 4 drivers provide faster performance and direct access to DBMS features.