What is the syntax for adding comments in CSS?

The CSS specification spells this out definitively:

Comments begin with the characters /* and end with the characters */. They may occur anywhere between tokens, and their contents have no influence on the rendering. Comments may not be nested.

CSS also allows the SGML comment delimiters (<!-- and -->) in certain places defined by the grammar, but they do not delimit CSS comments. They are permitted so that style rules appearing in an HTML source document (in the STYLE element) may be hidden from pre-HTML 3.2 user agents. See the HTML 4 specification ([HTML4]) for more information.

For example:

.row {background-color:#FFF;}  /* normal */
.row:hover {background-color:#CCC; /* grayed background */}
 
/* the title class */
.title{
    /* make font bigger */
    font-size: 160%;
}