WHAT IS KEYGEN???
A keygen (an abbreviated form of "key generator") is a small program that will generate valid CD keys or serial/registration numbers for a piece of software. These are made available by software cracking groups for free download on various websites dedicated to software piracy. In some countries, the use of keygens to activate software without purchasing a genuine code is unlawful.
Advantage of key-gen
The advantage of using keygen is that you can install software and finding its serial keys on web or use by the legitimate owner.
Dis-advantage of keygen
The dis-advantage of keygen is that you cannot use it if you don't have serial keys. Even if you can install softwares without the keygen your software will expire in the following days.