Hello Middle Seat!
United Airlines announced today that they were following Delta's lead and would be offerring so-called "Basic Economy Fares". When Delta started offering similar fares in the summer of 2015, they offerred a savings of between $10 and $25 per flight, on average. So they're definitely cheaper, but boy can they be ugly.
Buy one and you'll be assigned a seat, with no choice whatsoever (and that will also go for parents traveling with children, apparently). You won't get points that you can use towards upping your status with the airline, though you will get some other form of points. (How the points will differ is unclear). You'll be the last one allowed to board. But that won't matter because roll-aboard suitcases will be verboten for those with basic fare tickets.
Buy this most affordable seat and you'll only be allowed one carry on, and it will have to be of the small sort that fits under the seat—the overhead bins will be unavailable to you.
That last point is important as it means that you'll likely have to check a bag, so do check what happens to the overall fare once you factor in the fee for baggage. On most airlines, the baggage fees alone are $25, which could wipe out any savings on the final cost of the flight.
This new class of seats will go on sale beginning in January, for flights that start in March. Read the Associated Press' take here.