Cocoa Fundamentals Guide: Table

**Table 2-2** Important Objective-C defined types and literals
Type	Description and literal
`id`	The dynamic object type. Its negative literal is `nil`.
`Class`	The dynamic class type. Its negative literal is `Nil`.
`SEL`	The data type (`typedef`) of a selector. The negative literal of this type is `NULL`.
`BOOL`	A Boolean type. The literal values are `YES` and `NO`.