Taking a page from Bungie's difficulty naming convention, a fairly straightforward name, and a longer description:
Trivial - You will face bug-ridden software programmed by inebriated college students as an undergraduate computer science project. Military experts are still puzzled as to how it managed to defeat humanity in the first place. When you are inevitably victorious, you will feel as if you have defeated a small child.
Easy - Your opposition is a decent combat simulator suite used to train officer cadets before a wiring mishap led it to take over the galaxy. You are very likely to be victorious, unless you lack tactical and strategic flexibility.
Normal - The foe is a fully sentient artificial intelligence spanning across galaxies, but its resources are not infinite. If you are clever and resourceful, you may very well defeat the enemy and be rightfully proud of your accomplishment.
Hard - An ever-evolving enemy will test your skills to and beyond their limits with a bewildering array of deadly weapons and tactics. If you survive and are victorious, all will praise your undeniable strategic ability.
Impossible - Your opposition is the gods striding across galaxies, and you are but an ant scurrying about underfoot. The machine may crush you without even noticing what happened. If you win, it's probably because you cheated.