Long-tail keywords are more like phrases that people search for while head terms are shorter and more generic.
Long-tail example: Restaurants with outdoor seating in ann arbor mi
Head-term example: restaurants
Head terms are usually searched for more frequently, often making them harder to compete for and more difficult to rank for than long-tail keywords. However, the traffic you get from a more specific, long-tail keyword is often more desirable. Someone who is searching for “restaurants with outdoor seating in ann arbor mi” as opposed to just “restaurants” is probably a much more qualified searcher for your specific business or product. Someone just searching for “restaurants” could be searching for a host of reasons unrelated to your business.
At the end of the day, you should have a healthy mix of both of these. You want long-tail keywords that will award you short-term wins, while you can try to earn ranking for more difficult head terms over the long haul.