Well Streaming textures, are textures loaded when needed only in the fly.
Mega textures is creating (with specific tools i hope), a big texture containing all your level textures without siez limit like a big big File (like the old WAD games file that contained all level and textures if i remember well ?? ) !

Well let's Conitec make it and see if there is someone that will find a good use, not only bad textures put together !