The AirPort Extreme and the Time Capsule both add an interesting option to a network: they can share disks across a network without those disks being attached to a computer. Both models can accept one or more external drives plugged in via USB or via a USB hub; the Time Capsule also includes a non-removable internal drive.
Either model can share drives over a network with both the standard Apple Filing Protocol (AFP) format, the same format used with Personal File Sharing and OS X Server share files, and Samba, a network file-sharing service compatible with Mac OS X, Windows, and Linux.
Attached hard drives can be accessed over the Internet via AFP using Back to My Mac, too (see Access a Base Station via iCloud.
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