I found this explanation on OsX's portable home directories good (warning - you really need OsX server 10.5 to use this):

Tiger Server introduces "portable" home directories. I've been setting up accounts with this technology a bit more lately. Here are my observations, and a workaround to the ~/Library issue.

Overview Windows, coupled with a Windows Server, has a technology known as the "roaming profile." This basically takes most of your files from "Documents and Settings" plus appropriate registry settings and stores everything on the server. When you login to a Windows workstation, it grabs your profiles from the server and makes a local copy. When you log off, it copies your current set up back up to the server. All in all, this works relatively well.

OS X Panther Server has a technology called "mobile home directories." Most thought this would rival the Windows setup, but it didn't quite. The Panther based mobile home directory doesn't sync your directory to the copy on the local machine. Of course, Panther and Tiger have Network Home Directories, where once logged in, you work directly off of the server. This works great, even on a 100BT network, depending on how much data you're pushing around.

However, this doesn't help out laptop users. Enter portable home directories, introduced with Tiger. A managed network account can be set up with a preference to "Synchronize account for offline use."

The portable home directory is a little bit of a hybrid between Window's roaming profiles and Panther's mobile home directories [From OS X Tiger Server: Mobile, Portable Home Directories | Radiotope]

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