Microsoft Expression Media (derived from the iView Media product acquired by Microsoft in July 2006 and available for the Mac as well) is the Expression suite's organisational tool for still images, video and audio.
In some ways Microsoft Expression Media is like a bigger, more professional cousin to Windows Vista's Photo Gallery application, not only because it supports more than just pictures but also because it works with high-end media types - such as RAW-format camera files from many major manufacturers such as Olympus or Nikon.
Adobe's competing product in the Microsoft Expression Media arena is probably Lightroom or Bridge, both of which contain organisational tools and work with RAW image files as well and have a deeply entrenched user base that's not likely to simply switch on a whim. On the other hand, Lightroom and Bridge don't work with anything except still images.
A more direct Microsoft Expression Media competitor would be Extensis Portfolio, which supports RAW formats and video and offers a great many other professional-level features (such as a server component for the asset catalogue). Right now Media isn't much of a challenger, but it is a cautious first step and a decent starter choice for people who have no application like this yet.
The first thing you'll want to do with Microsoft Expression Media is fill it up with, er, media. This is easy enough: Select File, Import Items, point it at a directory, and let it slurp everything up. The files aren't moved from their original location; instead, a catalogue is compiled from their path names and metadata, and the catalog can be saved anywhere.
Once Microsoft Expression Media figures out what's to be imported, it processes the files asynchronously - you can continue doing other work with the program while it buzzes through everything and categorises it. Any folder added to a catalogue can be monitored for future changes (additions or deletions), although the flexibility of this feature is a little limited - you can set it to run only once a minute, once every five minutes, or on demand.
The next big thing you'll probably do with imported media is categorise it and add metadata - a task that ought to be familiar to anyone who's already played with Photo Gallery. If you add metadata to a file that is supported by industry-standard extensions for that file type - such as EXIF - it's written back to the file in addition to being stored in the catalogue. File types that cannot support some types of metadata directly (such as PNGs) will just have their metadata written into the catalog.
Expression Media's interface for adding metadata is far more detailed than Photo Gallery's, but we miss a few things from Photo Gallery that would be handy here. For instance, when you add keywords to existing pictures by typing, Media doesn't use smart completion to prompt the user from the existing store of keywords; it prompts you only from whatever has been typed in during that session. (On the plus side, you can assign keywords by dragging and dropping pictures onto a given keyword.)