A pseudo file system does not exists on the hard disk, it exists only in RAM when the system is running. These are primary pseudo filesystems in Linux:
/proc
Contains information about the processes running on the system. Processes are listed by PID in this directory along with some additional hardware information (cpuinfo, meminfo, partitions etc.)
/sys
Contains information about hardware and kernel modules
/dev
Contains actual handles to all devices that are connected to the system. Any piece of hardware thats get picked by udev, which is actual device manager, gets added to /dev. The udev works in conjunction to D-Bus which is data bus service that passes all information about everything going on within system to other services and devices.