I usually think of the description as an overall view of the story like a back cover to a book. The foreword, to me, is like allowing the reader to flip the page and see what else is there and why they should read the story. That's usually where I add an excerpt from my story to allow my potential reader to get a view of my story in action. =)

theoretically all these are very good answers but you know what it all comes down to? your description should be able to make potential readers click on your story title. forewords should hook them in enough to make them click next. so you do you and put what you think you can write best under each heading, cos there's no absolute answer to this!

but yes of course, to first catch a potential reader's attention you should like at least give a v short and interesting summary of the plot since only a portion of descriptions show before you click the story, and forewords should then explain more-- be it with an extract from the story, your personal notes about it, and whatnot. in the end the goal of these two is the same tho, to hook in as many potential readers as possible. that's the way i see it.

From what I understood:
Description: plot
Foreword: an extract of the story + every random things you want to tell people

But the truth is most people write whatever wherever XD

Description is sorta like a summary for the fic, but not really, if that makes any sense. Its like a blurb if you will, supposed to get the reader interested, but without giving away too much of the story. Foreword is basically an author's note or anything the author would like to say about the story. Hope this helps! ^^

description - the basic plot/short summary of your story without spilling any spoilers.

foreword - an introduction to the story. i'm guessing this is where you can tell readers what the genre will be, what idols/characters you'll be using, and authors thoughts like what inspired you to create this story, etc.