Actually it is hard to tell. Every application is simple from start, but then expand.

You could say Instagram is simple - create a user, upload a picture which people can see if they choose to add you as a friend. Simple at first, but then the application evolves to filters and sophticated algorithms for finding best content

They can be simple like write.as or complex like wordpress.com, Blogger etc. Simple as in the just write kind of thing, and complex as in there are probably too many extra features.