Sorry for not elaborating on what it is. You have to download the app because the web is just a view on some of the content. It's an alternative, pseudo-anonymous social network with a LOT of users (though not generally the demographic on YC). It's not a meme site / image macro site at all - it's just hard to see what it is the same way it was hard for people to understand Twitter for a long time after it launched. (Is it a blogging site, is it an RSS replacement? etc)