Saju and bazi are traditional East Asian systems of destiny reading that analyze a person's birth date and time to reveal character, life patterns, and fortune. Both map your birth information onto four pillars of heavenly stems and earthly branches. Bazi, in Chinese, means Eight Characters. Saju, in Korean, also means Four Pillars, and its full name, saju palja (사주팔자), also means Eight Characters.
Are they the same thing? Pretty much, I'd have to say — with some potential minor differences.
What we know for sure
Even those who consider saju and bazi to be separate need to admit that the core systems are identical. The reading is done the same way — same stems, same branches, same ten gods, same five elements. Like much of Korea's classical intellectual tradition, this system has Chinese roots. It arrived in Korea through centuries of cultural exchange between two neighboring civilizations with a long, complicated history.
What might be different
Some people believe that Korean practitioners emphasize fate and character more than Chinese practitioners, or that Chinese practitioners tend to focus more on practical outcomes like wealth and career. This could be true, and it makes sense. Different cultures have different ways of doing and seeing things. But whatever these differences may be, they vary by individual practitioner, and they're not enough to claim that these differences constitute separate systems. There is probably a bazi master who reads very similarly to a saju master, but would we say that they're actually doing saju? Probably not.
Another potential difference
There's one edge case worth noting: If your birth time falls between 11 pm and 12 am, you'll see that the reading on Moon Pavilion differs from most bazi sites. That's because my saju sources treat the 자시 (Rat Hour, 11 pm–1 am) as belonging to the following day rather than the current one — meaning a birth at 11:30 pm on March 15th would have the Day Pillar for March 16th.
This interpretation doesn't appear to be common in the bazi sites I've seen — in fact, I have yet to come across a bazi calculator that does it this way. I genuinely don't know whether this is a saju-specific convention, or simply varies by practitioner on both sides. If your chart looks different here, this is probably why. If I find out more about this, I'll certainly update you.
Why I call this a saju site
Simply put, it makes sense for me to refer to this practice as saju, which has been practiced by my ancestors for many centuries.