What does the catalogue refers to in the annotation definition for @Table?
This is related to databases. From EJB3 In Action, "you can think of a catalog as a "meta-schema" or a higher-level abstraction for organizing schemas. Often, a database will only have one common system catalog".
Also what does the @UniqueConstraint at a class level signify?
At a class level ? What do you mean by that ? @UniqueConstraint is used in the @Table annotation. From the API : "This annotation is used to specify that a unique constraint is to be included in the generated DDL for a primary or secondary table."