Different types of views

Views:
A view is a logical view on one or more tables, that is, a view is not actually physically stored, instead being derived from one or more other tables.
Types:
Database views
The view defined in the ABAP Dictionary is reproduced in the underlying database. You can use both ABAP Open SQL and ABAP Native SQL to access such views from ABAP programs, but you can only define them using transparent tables. If you define a database view using only one table, you can make changes to the view. For database views containing several tables, however, only read accesses are allowed.
Projection views
Projection views are used to suppress certain fields from a table in the interests of minimizing interfaces. A projection view can only refer to one table and, in contrast to database views, you cannot specify any restrictions with regard to table type. This view type also permits both read and write accesses with ABAP Open SQL.
Help views
Help views are used as the selection method of elementary search helps if the selection is too complex to be defined with a single database table. In contrast to database views, help views implement an outer join. For this reason they are suitable for linking supplementary information such as explanatory text from secondary tables. If the supplementary information were missing in an inner join, the entire dataset would not be selected.
Maintenance views
Maintenance views provide you with a business view of the data. You can change it either with the table maintenance transaction SM30, which allows you to maintain data from the base tables in a view at the same time, or with the customizing transaction. The mechanisms for data maintenance such as screens and processing programs can be created with a special transaction (SE54).