Cheating is playing the game while giving yourself an advantage using an external tool.
Methods of cheating
Playing a hacked version of the game
Various websites have hacked the game to do things like make upgrades free or give you money. Playing on these websites is risky; make sure your antivirus is up-to-date. It also is a copyright violation that deprives Kongregate and the game's author or revenue. Note the game tends to crash when you push its limits like this.
Using an autoclicker
Download a program that simulates manual clicks on the current ore. Be sure to disable it before you upgrade, to avoid buying upgrades you don't want.
Various utilities can cause other buttons (keyboard, scroll wheels, etc) to produce more clicks than normal clicking.
Before you're about to craft, load up the game in multiple tabs. Craft in each one. Close all the windows except the one that got you the best pick.
Influencing/viewing the random numbers that get generated
Use a program that "loads the dice" of the random number generator so that you can control the results of crafting. Or, view the random number that's next to be generated so you can craft when it's best. (Though, practical use of this requires understanding the actual algorithm used to generate a pick)
Altering the flow of time
Change Flash's perception of time passing to make idling happen more quickly. (gets you more gold, gems, and idle power without clicking)
Setting your date to a time in the past, then closing the webpage, then setting your date to a time in the future, to earn more gold and idle power multiplier while the game is closed.
Edit the program's variables directly
There are various utilities that can edit the memory of the Flash object and change the variables.
The point of playing a game is to enjoy yourself. If you get more enjoyment from cheating, as long as you don't harm or deceive anyone else, go right ahead. For most people, cheating makes the game less enjoyable in the long run. This may be a desired effect if you want to be done with the game sooner.