Synonyms: All of these nouns refer to something that prevents action or slows progress. Obstacle applies to something that stands in the way: "We combat obstacles in order to get repose" (Henry Adams).
An obstruction makes passage or progress difficult: A sandbar is an obstruction to navigation.
Bar and barrier suggest an obstruction that confines or prevents exit or entry: "Tyranny may always enter—there is no charm, no bar against it—the only bar against it is a large resolute breed of men" (Walt Whitman). "Literature is my Utopia.... No barrier of the senses shuts me out from the sweet, gracious discourse of my book friends" (Helen Keller).
Block suggests obstruction that effectively prevents all passage: I had a mental block and couldn't remember the date.
Hindrance and impediment are applied to something that interferes with or delays passage or progress: "an attachment that would be a hindrance to him in any honorable career" (Thomas Hardy). Overcrowded classrooms are an impediment to learning.
A snag is an unforeseen or hidden, often transitory obstacle: Due to a snag in plans, the project was delayed.