The duration of the critical path determines the duration of the entire project. Any delay of a terminal element on the critical path directly impacts the planned project completion date (i.e. there is no slack on the critical path).
A project can have several critical pahts. A path just shorter of the critical path is called a sub-critical path.
Originally, critical path method considered only logical dependencies among terminal elements. A related concept is critical chain, which adds resource dependencies.
The critical path method was invented by the DuPont corporation.