The most frequently asked question people have about this sort of thing is, "what's a jet generation." Simple, it's just the periods during which jet fighter technology made major advances. Thus the first generation jet fighters were those produced during World War II and through the late 1940s. The best examples of these are the U.S. F-86 and the Russian MiG-15. The second generation got going in the early 1950s, and produced aircraft like the U.S. F-104 and the Russian MiG-21. The third generation followed within a decade, producing the U.S. F-4 and the Russian MiG-23. The fourth generation arrived in the 1970s and 80s with the F-15, F-16, F-18, MiG-29, French Mirage-2000 and Su-27. The fifth generation includes the F-22, F-35, Eurofighter and Rafael. Russian fifth generation fighter developments were halted when the Soviet Union disintegrated in 1991. Actually, all development work on new fighters, by everyone, slowed down in the 1990s. But work on the F-22, F-35, Eurofighter and Rafael continued, and those aircraft became, in roughly that order, the most advanced fighter aircraft available today.
Actually, the Eurofighter and Rafael are considered more "generation 4.5" than fifth generation. The new Russian aircraft will have to provide some big advances in electronics or stealth to earn a place in the fifth generation. Otherwise, the Russians are just developing more generation 4.5 aircraft. But if the Russians can provide inexpensive new fighters, they will find a market for them.
Without providing much detail, Russia announced that it is developing two fifth generation fighters, to compete with the American F-22 and F-35. The MiG company will do the workon the light fighter, Sukhoi will do the heavy fighter. A new, thrust vectoring, version of the AL-41F jet engine is being developed for these fighters. New electronics systems are under development as well.