Multi line comments span over two or more lines and are ignored by the compiler.
In C they start with a slash and an asterisk /* and they end with an asterisk and a slash */.
In this case we do NOT need a semicolon at the end of the line.
Example:

#include <stdio.h>

int main()
{
    /*
    This is
    a multi
    line comment
    */

    return 0;
}

Single line comments span over one line only and are ignored by the compiler.
In C they can be done with a double Slash.
In this case we do NOT need a semicolon at the end of the line.
Example:

#include <stdio.h>

int main()
{
    // This is a single line comment

    return 0;
}