The American military is moving towards a single fighter plane, designed to be versatile enough for all combat situations.
The design includes VTOL functionality, meaning that it can take off and land vertically (because the Marine's really like this feature).
However, including this means that the plane is heavier, slower, less maneuverable, and less safe than current models.
In addition, the Chinese have build a copy of the plane that is better in almost every way, simply because they chose not to include VTOL functionality.
The F-35 will cost the U.S. government around a billion dollars, and probably end up being less effective than current fighters.