Hi, I have a PhD. Let me try an answer.

The tldr is that undergraduate focuses on learning, and graduate/PhD focus on discovering.

Refer to this diagram that others have linked: https://www.reddit.com/r/PhD/comments/u65rnp/a_phd_explained_in_a_few_diagrams/?utm_source=share&utm_medium=android_app&utm_name=androidcss&utm_term=1&utm_content=share_button

Long version below:

In undergraduate degree, your primary job is learning. For the first 2 years, think of it as more difficult highschool -- a general, broad education. In the last 2 years of undergraduate, you'd pick a major and focus heavily on its selected courses -- imagine taking 4 hours of biology classes everyday. Similar to your previous educations, your performance is evaluated on whether you score well on a test -- things that the teacher knows an answer ahead of time. You're acquiring the accumulated knowledge of humanity.

Graduate degree has two levels. You either do a masters (2yrs typically), or you continue after masters to do a PhD(3+ more years on top). I'll explain PhD first.

In a PhD program, your primary job is discovering. Unlike all previous education, in a PhD program nobody knows the answer ahead of times. Your "exam" is more like a class project on steroids: years of research in testing a hypothesis (that nobody has thought of before) by running experiments, and writing up your findings in a scientific paper. Your paper is evaluated by a group of scientists expert in the field (called peer reviews). Once you're done enough original research, preferably with paper publications, you write up a thesis summarizing your original works and graduate.

You know the term "scientist" right? During and after a PhD is when you get to call yourself a scientist, as you'd experienced what it's like pushing the boundaries of human knowledge.