I was a CFI and sales demo pilot in the Cherokee 140 for a Piper Flite Center in the early '70s, and was a partner in one of the last 140s built (a '77 model), so I know them pretty well. The Cherokee 140 is a safe, comfortable (in front, anyway) and inexpensive first airplane to own. Performance is very similar to a 150-hp C-172, but it doesn't do as well on short fields and struggles some above 10,000'. Gross weight and useful load are both about 150 pounds less than a comparable 172. My '77 140 (well-equipped) had a useful load of 730 lb. On the plus side, a 140 will carry 50 gallons of fuel, giving a nice, long range, if there's not too much weight in the cabin. The 140 worked out well for us as a first airplane for a family with two small kids.
In the early 1970s the Cherokee 140 was marketed as a fleet flight-school trainer, with optional snap-in rear seats for occasional use; while the C-172 was sold as an entry-level family/business traveling machine.
Though the Cherokee 140 exterior dimensions and appearance are identical to the full-four-seat Cherokee 150, 160 and 180, the 140's rear seats and baggage area are smaller than those models. Piper needed a trainer to compete with the smaller, cheaper Cessna 150, so in 1964 they took the Cherokee 150, removed the rear seats, moved the rear cabin bulkhead forward by one station, and sealed up the baggage door -- and called it the Cherokee 140. In 1965 they offered the snap-in rear seats as an option, and called it the "Cherokee 140 2+2 Cruiser". The rudimentary back seats leaned against the flat rear bulkhead with no cargo room behind them at all. The net result was somewhat less rear seat legroom than in the Cherokee 150, which was discontinued in 1967.
The 1969 model, dubbed "Cherokee 140B", added a lot of improvements, including the six-pack instrument panel and throttle quadrant introduced on the Arrow in 1967. A new molded plastic rear cabin bulkhead was offered as an option, which provided a tiny baggage area and hat shelf behind the snap-in rear seats. But there was still no baggage door. A restryled nose cap was introduced late in the 1971 model year, and the 1972 model added a dorsal fin and small extension to the top of the rudder, along with optional shoulder harnesses. The '72 140 still had the quirky ceiling-mounted trim crank, a holdover from the Tri-Pacer days. Other Cherokee models had switched to a more conventional floor-mounted trim wheel by then, and the 140 joined them in 1973.
From 1971 to 1974, Piper offered a stripped-down, two-seat, fleet-spec version of the 140, called "Flite Liner", to its Flite Center network. It came with just basic gyro instruments, one navcom, the old flat rear bulkhead, no back seats, no wheel fairings, no outside cabin entry step, no toe brakes. The only options were a 360-channel com radio instead of the standard 100-channel unit, and blue exterior trim paint instead of red. Over the years many Flite Liners have been re-equipped with features otherwise seen on standard 140s, so it may be hard to tell if the airplane you're looking at started out as a Flite Liner. A registration number ending in "FL" is a clue.