A garden path sentence is one which figuratively leads the reader down the garden path, misleading him or her into thinking that the sentence’s meaning will be different than what it really is. Grammatically correct, a garden path sentence is misleading or confusing because when people read in English, they build meaning one word at a time, in accordance with their own experience and preconceptions. Most garden path sentences’ meaning is cleared up once the entire sentence is read, although the reader must often backtrack to re-examine and re-interpret the words.
Many English words do double duty as both nouns and verbs, and the ambiguity of many garden path sentences is based on that. For example, when reading a sentence that begins with “The old man,” people assume that “man” is a noun. When the sentence ends with “the boat,” though, readers have to change their assumption of the full sentence — “The old man the boat” — to have any meaning. Once we realize that “man” in this case is used as a verb meaning “operate,” the sentence’s meaning is clear.
Many sentences are ambiguous, but that doesn't necessarily make them true garden path sentences. A garden path sentence contains a local ambiguity, which is cleared up within the sentence, like "The old train, the young fight." On the other hand, "The cat was found by the shed by the gardener" is globally ambiguous because the meaning remains unclear.
Another feature of English that leads to garden path sentences is the propriety of leaving words out of sentences. “The horse that was raced past the barn fell” is a perfectly fine and grammatically-correct sentence with a fairly straightforward meaning, which is basically “the horse fell.” When the words “that was” are removed, though, the sentence is no less grammatically correct, and the meaning is unchanged. A reader encountering “The horse raced past the barn fell,” though, will most likely have to backtrack at least once to reconsider initial assumptions about the sentence’s meaning.
The misleading nature of a garden path sentence is much more pronounced when written. This is because there are so many more components of spoken communication, such as inflection and tone of voice, that the sentence’s actual meaning is often clear by the time the sentence has been spoken. Newspaper headlines are sometimes garden path sentences, such as “Ambulance Crew Helps Snake Bite Victim;” an example from World War II was “Eighth Army Push Bottles up German Rear.” Headlines like these are sometimes deliberately written in garden path style to grab the reader’s interest. After all, it’s the headline writer’s job to persuade readers to read the story under the headline, and a garden path sentence is often a reliable attention-grabber.