Russian modules presently connected to ISS. The Nauka module will be the base block of OPSEK.
After Salyut, Mir, Skylab, ISS & Tiangong, space will get a new space station OPSEK. The base module of the space station will be Russian Nauka module, which is yet to be launched for ISS. After the ISS retirement, the Russian modules will be assembled again to give rise to the new space station, OPSEK. Currently ISS has several Russian modules which were taken directly from Mir station.
Some of the Russian segments in ISS are:
ZARYA: Functional Cargo Block (FCB)
Zarya module is used for the propellant storage for the midcourse correction of ISS. Since due to the heavy structure, ISS required thrusters activity to maintain the attitude, and hence huge amount of propellant for the attitude control thrusters is required. These are supplied by Russian Progress spacecrafts basically.
Zvezda: Service module
Zvezda is derived from the Mir space station core module. This module contains a total of 32 attitude control & 2 orbital maneuver thrusters.
PIRS: Docking Compartment 1
Pirs serve as the docking port for the Russian spacecrafts including Soyuz, Progress etc. It also act as the Extra Vehicular Activity (EVA) for the astronauts
Piosk is used as the docking port for Soyuz and Progress spacecrafts. This module also act as airlock which can be used for Extra vehicular activities.
MRM-1:Mini Research Module
MRM -1 is Russian built module with modification in storage space and additional clearance for Soyuz spacecraft. It is connected to the Zarya module. Although it was Russian built, it was launched with US space shuttle in May, 2010.
Nauka: Multipurpose Laboratory Module
Russian laboratory module yet to be launched to ISS. It will be primary research module for Russian segment on ISS and future Russian OPSEK station.
European Robotic Arm
Russian robotic arm proposed to be launched for the Russian segment transport system.
Russia to get new Space Station...OPSEK