There's an interview (just getting to that stage is extremely hard). I think that over 50% of those interviewed make it. See the sibling comment for the application process.
Read up on successful applications (google "my yc application" or "succesful yc application").
Some things I believe help making into YC much easier (but not guaranteed):
- Female founders or co-founder
- Very fast and demonstrable traction
- Large potential market
- Founders with successful backgrounds
--
Stuff that won't help on its own:
- Interesting / world changing project (unless you have demonstrable traction). They say they invest in idea-stage companies, I believe that only happens with founders with huge exits on their backs.
Note that this is just my own opinion and I did NOT get into YC and I'm still a BigCo employee.
Sample succesful applications:
https://www.themuse.com/advice/the-muses-successful-applicat...
big list of applications and advice:
https://yourstory.com/2014/10/guide-list-y-combinator/
