What's the criteria for Padma Shri or Padma Vibhushan?

I must admit, though rather sheepishly, that all through these years I have never bothered to check just what the criteria are for Padma Shri or Padma Vibhushan. But this year I want to check them out.

The reason is that some of the names in the list of awardees didn't make sense to me. e.g. Madhuri Dixit's. It would have made sense if she had acted in a lot of meaningful movies or had made significant contribution to classical dance (she is a dancer, isn't she?). But how acting majorly in commercial run-of-the-mill potboilers has qualified her for Padma Shri is simply beyond me.

So, now about the Padma Shri or Padma Vibhushan criteria.....can someone please direct me to the appropriate web site? :-)