It "is a defensive obstacle, which existed in a number of forms and were employed in various applications. These included underwater constructions used to prevent the passage of ships or other vessels on rivers, or as anti-cavalry measure consisting of a portable frame (sometimes just a simple log) covered with many projecting long iron or wooden spikes or spears."
Cheval-de-Frise is a usually moveable object consisting of a wooden frame covered with spikes used to close off a passage or to block enemy advancement. During the Civil War, these were generally made of wood and used as defensive obstacles.
Edit - A defensive obstruction is an acceptable response to the second part of the question, but the response to the first part of the question, explaining how a cheval-de-frise differed from any other type of defensive obstruction, has been submitted in the form of a link, which is not acceptable according to the rules.
1) It is a defensive obstacle constructed by using a long horizontal beam pierced with diagonal rows of sharpened spikes.
2) It is a used as an effective barrier for roads and fortifications.
Note for hoosier: there are several descriptions out there telling what a Cheval-de-Frise is made from. Score the answers at your own discretion, as long as it is clear what is meant, sharpened spikes or blades or whatever that will block the passage.