Very few programming languages do not support comments. Comments are useful ways of adding contextual information about a piece of code in a program.
The difference between these two types of comments is that the first version only works across a single line:
//This is a comment
The second version can span multiple lines:
/* This is a comment * This is also a comment */
It is often the case to use an asterisk on each line between the opening and closing comment tags as shown on line 2 previously but it is not completely necessary.