what 's the use of sysfs filesystem?
what's the difference between sysfs directory and other directories?
we 're reviewing the fedora source and have problem undrestanding this type of file system (sysfs)

The sysfs Filesystem
The sysfs filesystem is a special filesystem similar to /proc that is usually mounted on the /sys directory. The /proc filesystem was the first special filesystem designed to allow User Mode applications to access kernel internal data structures. The /sysfs filesystem has essentially the same objective, but it provides additional information on kernel data structures; furthermore, /sysfs is organized in a more structured way than /proc. Likely, both /proc and /sysfs will continue to coexist in the near future.

A goal of the sysfs filesystem is to expose the hierarchical relationships among the components of the device driver model. The related top-level directories of this filesystem are:



block

The block devices, independently from the bus to which they are connected.



devices

All hardware devices recognized by the kernel, organized according to the bus in which they are connected.



bus

The buses in the system, which host the devices.



drivers

The device drivers registered in the kernel.



class

The types of devices in the system (audio cards, network cards, graphics cards, and so on); the same class may include devices hosted by different buses and driven by different drivers.



power

Files to handle the power states of some hardware devices.



firmware

Files to handle the firmware of some hardware devices.

Relationships between components of the device driver models are expressed in the sysfs filesystem as symbolic links between directories and files. For example, the /sys/block/sda/device file can be a symbolic link to a subdirectory nested in /sys/devices/pci0000:00 representing the SCSI controller connected to the PCI bus. Moreover, the /sys/block/sda/device/block file is a symbolic link to /sys/block/sda, stating that this PCI device is the controller of the SCSI disk.

The main role of regular files in the sysfs filesystem is to represent attributes of drivers and devices. For instance, the dev file in the /sys/block/hda directory contains the major and minor numbers of the master disk in the first IDE chain.

you can find more information in chapter 13 of this book
undrestanding the linux kernel

Sounds like homework.
Read ../Documentation/filesystems/sysfs.txt