Pathfinder fills two or three niches in the gaming world:
First, it's a system built around 3.5e Dungeons and Dragons, arguably the most popular of all tabletop rules systems. It aims to maintain the spirit of the system after WotC moved on with their newer versions and stopped publishing official 3.5e content.
Second, it aims to balance the game power creep to a moderate degree. It revised a bunch of rules that made 3.5e easily exploited, especially for spellcasters. It buffed the utility of a lot of combat maneuvers for melee types and gave them access to heaps of new tricks. It also introduced disincentives for creating the horribly mutant multiclass builds that were popular flavours of the month for 3.5e.
Finally, it makes the majority of its content open source, so it draws a big crowd of players simply because they can access the rules they need for free without buying a million splat-books, unlike 3.5e.
As a system Pathfinder still has all the crunchy-ness that 3.5e veterans love, but also a simplistic front-end that's not so intimidating for new players. It's not perfect and can still be terribly broken by the right wrong player, but overall it's a fun way to spend a weeknight with friends.