Trend is the collection with the peachy/nude label in the back of the neck, and it's the 'runway' collection. Most of the more risky fashion pieces come into this collection, and generally it's where most pieces feature in the press come from. It's only available at select stores though.
Divided Black is part of the 'Young' department, it's the garments with the long, thin black tag in the back of the neck. It's fairly young and sometimes a bit punky? Skinny pants, little pleather biker jackets, vests with prints on etc. It's quite cutesey sometimes.
Divided Red is the other 'Young' concept. It's sort of a bit more street inspired, and has a lot of hoodies, and generally more relaxed pieces really?
Then the womenswear department is split into Hennes and Modern Classic. They're the most widely available departments really, most stores stock them. Modern Classic is supposedly more workwear, while Hennes is your everyday womenswear.
Think that's about right... Most stores stock both womenswear departments and Young departments, it's just Trend that's particuarly tough to get hold of sometimes.