For me, when I hear people using the term smut with regards to writing, the first thing that comes to mind (and I mean no offense) is an IM-based RP where you literally just do a hot and heavy, very graphic sex scene and nothing more. No character development, no story/plot, nothing. Just the sex and it's over.
So, when people refer to stories as smut, I tend to quirk a brow. Sure, some stories have sex in them. Some have a lot. But, for me, if there's a plot and there's character development, it's far from smut in my eyes. Then, I'd either consider it romance or erotica depending on the level of graphic sex as well as what sorts of sexual encounters and the like were incorporated.
That said, to me, I see smut as something that's a mere quickie one shot of a particular scene. Or, to put it plain... text-based porn.
Also, let me add, nothing wrong with smut. It has it's place for sure. It's just, I don't think of it as something that's a true blue story or anything like that.