hibernate.dialect

Dialect means "the variant of a language". 
Hibernate, as we know, is database agnostic. It can work with different databases. 
However, databases have proprietary extensions/native SQL variations, and set/sub-set of SQL standard implementations.
Therefore at some point hibernate has to use database specific SQL.
So Hibernate uses "dialect" configuration to know which database you are using so that it can switch to the database specific SQL generator code wherever/whenever necessary.


In simple

hibernate.dialect property helps Hibernate to generate the appropriate SQL statements for the chosen database.

List of Hibernate SQL Dialects
http://docs.jboss.org/hibernate/orm/4.2/javadocs/org/hibernate/dialect/package-summary.html