Answer 1

Important is contextual, depends on whether the use of those symbols were syntactically correct. Now about the symbols;

* Semicolon ;
Is used as command separators, each command given to be processed are delimited by this symbol.

* Comma ,
This one is particularly uneasy to explain, as it could act as an operator or as separator (in variable declaration or function parameters declaration / passing arguments). The following links may provide you better explanation 👇

http://www.c4learn.com/c-programming/c-comma-operator/

https://en.cppreference.com/w/cpp/language/operator_other#Built-in_comma_operator

* Hash tag #
A line beginning with this symbol is called a preprocessor directive. This allows (for example) conditional compilation of the code, but there are more use cases. The following tutorial page can provide better explanation including some examples; worth a read.

http://www.cplusplus.com/doc/tutorial/preprocessor/