Semantics
We do not need to use artificially complex BNF definitions!
Derivation trees are the real sentences of the language.(Strings of symbols are just abbreviations of trees, these abbreviations may be ambiguous)
Concrete Syntax- determine derivation tree from string abbreviation (parsing)
Abstract Syntax: Analyse structure of tree and determine its semantics
Tree generated by concrete definition identifies a derivation tree for the string in the abstract definition.