The Black Death of 14th century Europe was caused by the spreading of the bacteria Yersinia Pestis on fleas carried by rats on trading ships from Asia. In Asia (where the plague had an even larger effect than it did in Europe), infected fleas would stay on rats, and the rats would board trading ships. These ships would land in European harbors, letting off the goods and the rats. The fleas they carried, along with the extremely cramped living conditions within cities at the time, caused the outbreak.
The Great Famine, I know less about, was caused mostly due to poor weather conditions and inadequate agricultural techniques. During that time period, famines were common; the Great Famine happened to be extremely impacting on society due to the widespread crop failures that ensued.
As a result of these two catastrophes, along with many others during the 14th century, society completely changed. The main factor was the population: nearly 2/3 of the Europe's population was killed during the time period. This meant that wages were raised for workers in urban areas due to the smaller number of people competing for work. Also, social mobility became easier for peasants due to less competition.
