Keygen is abbreviation of CD Key Generator. It can generate serial of a commercial program like Photoshop. Try not to download keygens because they are powered by viruses to generate CD keys. Some keygens are fake, they are just a random number generator.
I searched the serial of Photoshop on Google because keygens contains Trojan viruses.
An executable computer program which is used to illegally bypass copy protection on games and commercial software by generating a random serial number, or "cd key", that matches the software it is intended to be used with. These programs range from very basic to quite complex, and are commonly distibuted along with warez. However, most games with online capabilities can now determine if you are using an invalid key and block you from playing over the Internet.
"I lost my CD case for Call of Duty, so I had to download a keygen to get a new serial."