What are 'tumblr girls'?

Recently I've been on instagram, and I've been seeing accounts that are 'tumblr' they are just holding things up in front of a really good quality camera. More of canon rebels and nikons. Am I the only one that thinks that this is a waste of good camera? Sorry but I just really wanted to know what exactly it is.

They are easy to hate.
http://www.urbandictionary.com/define.php?term=tumblr%20girl

It looks like those girls that obsess over their looks and love lording it over us ordinary folks.
http://www.wikihow.com/Become-a-Tumblr-Girl