There are three most commonly used methods of Data adapter:-
Fill: - Executes the Select Command to fill the Dataset object with data from the data source. It can also be used to update (refresh) an existing table in a Dataset with changes made to the data in the original data source if there is a primary key in the table in the Dataset.
Fill Schema: -Extracts just the schema for a table from the data source, and creates an empty table in the DataSet object with all the corresponding constraints.
Update: -Updates the original datasource with the changes made to the content of the DataSet.