<!--[if lt IE 8]><![endif]-->
The lt in the above statement means less than, so 'if less than IE 8'.
For all versions of IE you can just use
<!--[if IE]><![endif]-->
or for all versions above ie 6 for example.
<!--[if gt IE 6]><![endif]-->
Where gt is 'greater than'
If you would like to write specific styles for versions below and including IE8 you can write
<!--[if lte IE 8]><![endif]-->
where lte is 'less than and equal' to