Flickr is asking users to specify the kind of content that they are uploading to Flickr servers. When you upload a new image to Flickr, you need to specify whether it's an art-work, photograph taken from a camera or a computer screenshot.
Flickr is also asking users to assign movie-style ratings to their photographs at the time of upload. The picture ratings can be Safe (G), Moderate (PG, PG-13) or Restricted (NC-17) - (though Flickr won't allow Porn pictures, nude images are permitted and hence the Restricted rating)
It is important that you set the content type else your pictures may not turn in the Flickr search engine or users may not be able to view them without logging into Flickr.
If doing this for each picture sounds too much effort, Flickr has a simple page where you can set the general preferences for your pictures. Later, when you upload a picture of different type, just specify that at the time of uploading.
Tip: If you have reached flickr while looking for screenshots, goto the advanced search page, deselect photos and select "screenshot" in the search option. That should give pretty relevant results.
Flickr URLs - Dissecting the web address of Flickr Pictures
Related: Shoot and Upload Screenshots to Flickr