Final answer:
Prokaryotic cells and eukaryotic cells are distinguished by the presence of a nucleus; eukaryotic cells have a nucleus and membrane-bound organelles, whereas prokaryotic cells do not.
Step-by-step explanation:
The characteristic that best distinguishes prokaryotic cells from eukaryotic cells is the presence of a nucleus. Prokaryotic cells lack a nucleus and membrane-bound structures, whereas eukaryotic cells have a true nucleus, in which their DNA is surrounded by a nuclear membrane. Additionally, eukaryotic cells contain other membrane-bound organelles that compartmentalize various cellular functions, allowing for greater complexity and specialization.
All cells, whether prokaryotic or eukaryotic, share four common structures: a plasma membrane that serves as a barrier between the cell's interior and the external environment, the cytoplasm which is a jelly-like substance containing other cellular components, DNA that is the genetic material of the cell, and ribosomes which are responsible for protein synthesis.