What's the main difference between prokaryotic cell and eukaryotic cell

Prokaryotic

they do not have a well-defined nucleus they are mostly unicellular

Eukaryotic

they have a well defined and membrane-bound nucleus they are mostly complex and multicellular

The main difference is that prokaryotic cells are singular: they don't form together to create a living being and are a form of bacteria. Eukaryotic cells do form together to create a living being such as animals or plants.

A good way to remember this is that Eu = You

You are a living being and are made of eukaryotic cells!