The Old Man and the Sea may be Hemingway's most read book, due to its short length (under a 100 pages) and simple plot and characterization. It was written late in Hemingway's career, and is in some ways non-typical of his work, while in some ways having very typical Hemingway themes.
The book has only one major character, the titular old man, Santiago, a Cuban fisherman who was at one time strong, but is getting weaker with age, but who still pursues fishing, perhaps out of economic necessity, perhaps because it is his lifestyle. Other characters include a young boy who helps Santiago, and a giant fish. And, in the character's thoughts, Joe DiMaggio. More biographical detail is given about Joe DiMaggio in this book than about the main character.
After a long period of being unable to catch anything, the old man captures a very large marlin, who carries him outwards into the ocean over several days. Santiago eventually kills the fish, but since it is too large to fit into his boat, he ties it to the side. Over the next few days, despite Santiago's best efforts, the fish is eaten by sharks. He returns to his home village with only the head of the fish.
The book is very simple, in the good way. It is very easy for just about any reader to identify the basic character of Santiago, and to follow the action. This is one of the reasons why it is a favorite book to be assigned in high school English courses: it is much more realistic for a high school student to follow the action of The Old Man and the Sea than (for example) Pride and Prejudice.
But while it is simple to follow what is going on, there are a lot of possible meanings to be taken out of the work. Some of the more obvious ones are the relationship between man and nature, the idea of destiny and fate, the religious symbolism of suffering and the idea of pursuing a duty regardless of results. There is also some obvious political or social commentary possible on how Santiago struggles to get ahead with no results. Pretty much any symbolism you can imagine can be gotten out of this short novel, if you so desire.
The Old Man and the Sea is a classic of economy, which despite its seemingly sparse story, can be read many different ways.