In addition of device nodes explained in other answers, notably the one from Sepahrad Salour, Linux has other "magical" files provided by special virtual file systems, in particular in /proc/ (see proc(5)) and /sys (read about sysfs).
These pseudo files are a virtual view provided by the kernel; in particular, reading from /proc/ (e.g. with cat /proc/self/maps, or by open(2)-ing /proc/self/status in your program) generally does not involve any physical I/O from disk or network, so is quite fast.