To most of us, art is just art. It's fun to look at, and fun to make. Most of us are pretty bad at it but some people are very talented when it comes to art. Other people, such as those in the field of Art History, are experts. They could walk you through important events throughout history (even ancient history) and explain the corresponding trends in art. They could look at any painting from a significant time in art history, and tell you who made it based on brush strokes, color selection, and style. It's a pretty amazing field, and education on the topic is available online and at community colleges and universities all over the place.
For the rest of us non-experts in art history, it's just fun to look at, but we probably couldn't tell the difference between eras or artists when we look at works of art. Yet art historians insist that each distinct painting, artist, and historical era had major differences.
So for us non-experts, to prove our point that eras in art history are still similar to today, take a look at these historical paintings captioned by Medieval Reactions on Twitter.