JDBC and Connection Pool are about database but both different from each other. Let’s start on what is Connection Pool.
Connection Pool – Is a database cache that is administered by the application server. It is the quick connection to the database, where the cache will handle the request from the website and not directly to the database. Caching is an important component for website performance.
JDBC – Is a Java API that provides database access. For Java applications that require database access, It must use the JDBC API, or a third party framework that has JDBC. Java web applications are deployed by a servlet container such as tomcat, or running as sole client application, which may not use a Connection Pool. But if the web application is deployed on a application server ( Webphere / Weblogic / etc .. ) , then it will use a Connection Pool.
wikipedia link on connection pool
wikipedia link on JDBC