A device is a term used mostly for hardware-related activities in a system, including disks, printers, graphics cards, and keyboards. When FreeBSD boots, the majority of what FreeBSD displays are devices being detected. You can look through the boot messages again by viewing /var/run/dmesg.boot.
For example, acd0 is the first IDE CDROM drive, while kbd0 represents the keyboard.
Most of these devices in a UNIX® operating system must be accessed through special files called device nodes, which are located in the /dev directory.
When adding a new device to your system, or compiling in support for additional devices, new device nodes must be created.
The device file system, or DEVFS, provides access to kernel's device namespace in the global file system namespace. Instead of having to create and modify device nodes, DEVFS maintains this particular file system for you.
See the devfs(5) manual page for more information.