We are currently on the third generation (series) of IS.
1999 - 2005 IS200/300 (1998 - 2005 as the Toyota Altezza in Japan)
2005 - 2012 IS250/IS250C/IS220d/IS200d/IS F
2013 - IS250/IS300h/IS200t
Following Toyota's standard lifecycle each series lasts around 7 years with a major facelift mid-cycle. Minor cosmetic changes occur yearly.
Trim grades have changed with each series. There have been tweaks between the years, and some special editions, but roughly they are:
Series 1 IS200 (base), SE, Sport
Series 2 IS250 (base), SE, SE-L, Sport
SE, SE-I, SE-L, F-Sport
Series 3 SE, Luxury, F Sport, Premier
SE, Executive, Sport, Advance, Luxury, F Sport, Premier