What is the difference between judging ppl? and expressing your opinions?

Having an opinion is not all that different from judging someone. In fact, it is the precursor.

Having an opinion is to form an idea about something, all the while, acknowledging the possibility that your idea could be wrong. You leave a room for things to be cleared out. You do not draw conclusions, or even if you do, you don't declare a person guilty using these conclusions.

Judging someone is placing yourself in the seat of the judge in the court, and delivering a sentence. A firm and unshakable idea, backed up by conclusions that you've drawn by viewing someone and their case through your vision.

So, if you make your opinion so firm and unshakable, coming to conclusions about someone without knowing the entirety of the story, and deliver a sentence to their name - that's called judging.