The minutia of the algorithm is private, to prevent gaming it, but the major factors are very well known, and make perfect sense.
- utilization percentage (if you're maxing out your credit line(s) all the time, that's a bad sign)
- payment history (if you don't make payments by the due date consistently, that's obviously an indicator that you're risky to lend to)
- age of account(s) (having made consistent payments for 6 months naturally isn't going to look as good as having done so for 5 years)