In SQL, data refers to the information that is stored in a database table. Each column in a database table is required to have a name and a data type. The data type of a column defines what value the column can hold, such as integer, character, money, date and time, binary, and so on. SQL data types can be broadly divided into categories such as exact numerics, Unicode character strings, approximate numerics, binary strings, date and time, and other data types. The data type provides guidelines for SQL to understand what type of data is expected inside each column, and it also identifies how SQL will interact with the stored data. For example, if a column is specified as INT, SQL will show an error if any other value apart from an integer is inserted into that particular column.

Data in SQL is stored in a tree-like structure, with table data actually stored in a series of 8 KB data pages. A data page is the fundamental unit of data storage in SQL Server, and it is 8KB in size. When data is inserted into a SQL Server database table, it is saved to a series of 8 KB data pages. The data is physically stored in the database in a sorted manner based on the primary key column.

In summary, data in SQL refers to the information that is stored in a database table, and the data type of a column defines what value the column can hold. Data in SQL is stored in a tree-like structure, with table data actually stored in a series of 8 KB data pages.