Wikipedia has the answer. Read syntax (programming languages) & semantics (computer science) wikipages.
Or think about the work of any compiler or interpreter. The first step is lexical analysis where tokens are generated by dividing string into lexemes then parsing, which build some abstract syntax tree (which is a representation of syntax). The next steps involves transforming or evaluating these AST (semantics).
Also, observe that if you defined a variant of C where every keyword was transformed into its French equivalent (so if
becoming si
, do
becoming faire
, else
becoming sinon
etc etc...) you would definitely change the syntax of your language, but you won't change much the semantics: programming in that French-C won't be easier!