A SQL session is an occurrence of a user interacting with a database through the use of SQL statements. A session can be invoked using 
vsql or a JDBC application. In Vertica, the scope of a session is the same as that of a connection.
JDBC (Java Database Connectivity) is a call-level API that provides connectivity between Java programs and data sources (SQL databases and other non-relational data sources, such as spreadsheets or flat files). JDBC is included in the Java 2 standard and enterprise editions.
vsql is the Vertica implementation of psql, a character-based, interactive, front-end that is part of PostgreSQL and used by other database management systems. It allows you to type in SQL statements and see the results. It also provides a number of meta-commands and various shell-like features to facilitate writing scripts and automating a wide variety of tasks.
