Almost immediately after the first computers were invented, people wanted them to do as they command. This process has gone through four distinct phases.
The command line
This was the original way. The user types his command in its entirety and presses enter. The computer then parses it and does what it is told. There was no indication on whether the written command was correct or not. The only way to test it was to execute it.
An improvement to writing the correct command. The user types in a few letters from the start of the desired command or file name and presses tab. If there is only one choice that begins with those letters, the system autofills the rest. Modern autocompletion systems can fill in command line arguments, host names and so on.
This is perhaps best known from IDEs. When the user types some letters, the IDE presents all choices that correspond to those letters in a pop up window below the cursor. The user can then select one of them or keep writing. Internet search sites also do this.
Live preview with error correction
One thing in common with all the previous approaches is that the input must be perfect. If you search for Firefox but accidentally type in “ifrefox”, the systems return zero matches. Error correcting systems try to find what the user wants even if the input contains errors. This is a relatively new approach, with examples including Unity’s new HUD and Google’s search (though the live preview does not seem to do error correction).
What is the next phase in command entry? I really have no idea, but I’m looking forward to seeing it.