On March 6, 1869, Dmitri Mendeleev unveiled a remarkable discovery-slash-invention: the periodic table arranging the known elements. He showed it to the Russian Chemical Society, and chemistry was forever changed.
Mendeleev's discovery had profound implications. By arranging the elements mostly according to their atomic weight, noting that various periodic relationships occurred among elements, and grouping related elements together, Mendeleev's table predicted useful information about elements that had not yet been discovered. Although only 60 elements were on that original table, we can draw a straight line from Mendeleev's work to the periodic table we see everywhere today. (Note: Mendeleev's table was very text-heavy; the modern boxy design we know came about starting in the 1920s when Horace Deming drew it up.)
So how did Mendeleev come to create his periodic table, and how did it help chemists? Hank Green taught a great Crash Course Chemistry lesson about the whole thing, starting with Mendeleev's childhood. Settle back for 10 minutes, and learn a bit about something we often take for granted.
If you're looking for a shorter introduction, check out this TED-Ed lesson. You might also be interested to know that German chemist Julius Lothar Meyer independently discovered the same thing in 1870, after working on a similar idea for years. So it goes.