What does this calligraphy say? who is it by?
The words are "天行健，君子以自强不息。地势坤，君子以厚德载物" from 《周易》。

君子以自强不息 means one(君子) should keep on hard working and self-improvement.

君子以厚德载物 means one should take responsibilities and be open-minded.

This is a motto of man's behavior. "天行健" and “地势坤” are terminologies of 八卦, a kind of divination. The way of man's life is extended from the result of divination.

Although many people do not believe in 八卦, this motto still motivates people to improve their ways of life.