DTO stands for Data Transfer Object, which is a design pattern used in Java to transfer data between different layers of an application. DTOs are objects that transport data between subsystems, and their main purpose is to reduce the number of method calls between client and server by aggregating data. They are similar to data structures, but they dont contain any business logic and are designed to provide an optimal representation of data for the needs of each layer.

DTOs are typically implemented as POJOs (Plain Old Java Objects) or Java Beans. They can store data from a single source or from multiple resources, and they can either store complete data or a small amount of data from a source. DTOs are mainly used for reducing the number of expensive remote calls, and they prevent tight coupling between entities, such as the domain model and presentation layer of an application.

DTOs can be used to transfer parameters to methods and as return types. They are commonly used in a variety of use cases in Java applications, including reducing the amount of data transferred, improving performance and scalability, and separating data transfer concerns from business logic concerns.

In summary, DTOs are objects used to transfer data between different layers of an application in Java. They are designed to reduce the number of method calls between client and server, prevent tight coupling between entities, and provide an optimal representation of data for the needs of each layer. They are commonly used in Java applications to improve performance and scalability and to separate data transfer concerns from business logic concerns.