500 Startups is an early stage seed funding incubator based in Silicon Valley, CA. Founded by Dave McClure with his philosophy - "there are no rules", 500 Startups has no application process. It accepts companies through referral. In short, 500 Startups is all about growth, network and funding.
Y Combinator is a leading accelerator based in Silicon Valley, USA. It runs three month program (2 batches a year) in which it provides seed fund, networking & mentor-ship to selected startups. Getting accepted in YC program is considered itself an accomplishment. With its slogan "Make something people want", Y Combinator has one of the most influential voice in today's startup world.