Like other languages powershell also supports single/Inline comments line and multi line comments.
Starting with PowerShell V1 there's only # to make the text after it a comment.
In PowerShell V2 "<# #>" can be used for block comments (multi-line) and more specifically for SYNOPSIS, DEsCRIPTION,
NOTES, LINK help comments.
Example: Sinlgle/Inline comments
Example: Block comments (multi-line)
Example
<# this is a first line of comment
this is a second line of comment
this is a third line of comment
.
.
.
this is a last line of comment
#>
you can find best consolidated reference for comments for other programming languages http://en.wikipedia.org/wiki/Comparison_of_programming_languages_(syntax)#Comments