Exploits are design flaws, similar to glitches, that can be used for a practical gain. Exploits are like glitches in that they weren't intended to be done, or weren't anticipated to be done. Exploits are unlike glitches in that they are not the result of faulty programming, but rather the result of successful programming used in a way that the game designers didn't intend.
Glitches versus ExploitsEdit
Using glitches and using exploits can have similar, even identical results, but the reason why you get the results are completely different. When doing a glitch there is an error in the game script that causes the glitch. For example, if the Master Chief cloaks using the Envy Skull and gets a checkpoint while cloaked and saves and quits the game, when the checkpoint is loaded again the Master Chief will stay cloaked infinitely. When doing an exploit the game script runs fine, but a part of the game script is what causes the exploit to happen. For example, the game script says that if you don't have a Hornet in one part of a level, the game will spawn another one and on that new Hornet will be two Marines. If you keep getting rid of the Hornet, you can make Hornets and Marines spawn forever and gather thousands of Marines. This is exploiting the game script rather than glitching because the script worked the way it should, it's just that it wasn't meant to be used thousands of times.