A column always contains the same kind of information. For example, a table that has information about employees would have a column for last name and nothing other than the employee last names should be placed in that column.
A row is one instance of all the columns in a table. For example, each row in the employee table would contain, among other things, the first name and the last name for that employee. The columns in a table represent entities, relationships, or attributes.
An entity is a person, place, or thing about which the table contains information. The table mentioned in the previous paragraphs contains information about the employee entity. Each table holds only one kind of row. The relational model requires that each row in a table be uniquely identified. To accomplish this, you define a uniqueness constraint to identify each row in the table. For more information about primary keys, see Relationships Between Primary Indexes and Primary Keys.