From Joomla! Documentation
JTable is an abstract class that forms the basis for all database table classes. Some of the methods listed will be overridden by the child class so you should check the child class documentation for further information.
Each physical database table created should have a corresponding class derived from JTable to represent it. JTable provides many methods to make common manipulations to the table much simpler. For example, one of the most common operations you will need to perform is to read a table row into memory given a value for the primary key. This can be done easily using the load method. The table row can then be just as easily updated using the save method, which also performs any predefined sanity checks on the table fields.