PO: persistant object persistent objects, can be regarded as a database and mapping of the broad account of java object. PO is the simplest in a corresponding database table in a record number of records can be used PO collection. PO should not contain any of the database.
VO: value object value targets. Normally used for business data transfer between the layers, and PO is just the same data it contains. It should be an abstract object to the business, and the corresponding table, or may not, based on business needs. Personally feel that the same DTO (data objects), on the web transfer.
DAO: data access object data access objects, the objects used to access databases. PO and often used in combination DAO includes a variety of database operation. Through its methods, combining PO related to the operation of the database.
BO: business object business objects, Packaging, the business logic java object, by calling DAO, with PO, VO conducted operations;
POJO: plain ordinary java object no simple rules of java object, and I personally feel that it is not a level other things, VO and the PO should belong to it.