Most decluttering asks that you look more at utility and how often you use things. She asks if they spark joy, asks you to intuitively handle each item and be honest with yourself about it. She doesn't believe we should be surrounded with anything that doesn't. I'm sure there's room for useful items that aren't exactly joy-sparking, but you get the idea.
She doesn't think decluttering is a big project every year or so. You go big once, and if you do it "right," you'll have little maintenance to perform that way. There will always be some (think of clothes especially), but even basic cleaning will feel easier/more pleasant without so much "just because" stuff around us.
She has an order. You do clothes first (and I mean clothes from every part of your house--got to be faithful to the categories), then books, then papers, miscellaneous (most other things) and then finally mementos/sentimental items. She even has an order to do the clothes. If you're overwhelmed by the volume, she recommends that you start with the off-season stuff. The book goes into detail about all these areas with tips, examples etc. Eric and I really like her folding ideas for storing clothes--it's amazing how much more we can fit in the drawers (which means no clothing in bins under the bed for me now).
Her feelings about books are that if you have it to read "sometime" it means likely never. She thinks timing is a big thing with books. The only ones you keep are Hall of Fame books (that you actually reread) and what's on your plate for now/recent future. If things are stored on the floor, she thinks that they belong in a closet. Lots to say on documents, filing etc. She has an order on the "miscellaneous" (or everything else) category too.
She's not one for big organizing systems--that basically leads to more hoarding. Apparently shoe-boxes work for most storage. In terms of why we typically hold onto things, she gives reasons. But she also asks the reader to consider: are we attached to the past rather than living in the present? Are we afraid of the future? Stuff like that.
Inspiring read! I used it as a guide for my Summer declutter (just for my stuff--can't impose on the others).
One course that's quite afordable but different is Inspired Everyday Living if you don't want to go so extreme. They have a lot of great ideas for simpifying and really thinking about the energy of our spaces.