Distinction between statements, declarations, and expressions in formal grammar
These are informal terms. For example, in the C language:


Statements are things like x = 3;.
Declarations are things like int x;.
Expressions are things like x * y.


So declarations are a particular type of statements, and expressions appear in statements. 

A better formulation of the statement you quoted would be:


  Nonterminal symbols represent syntactic entities such as statements, declarations, or expressions.