Formerly known as FruityLoops, this application is a Digital Audio Workstation (DAW), and is used for the creation and production of music. There are a number of versions available to cater to different markets, from home hobbyist to professional. This app has been used by many notable and successful artists.
This application is released under what is known as a "crippleware" license. Essentially, this means that the software in its entirety is available to download for free, however the features available to you are restricted until the application has been registered, which means paying for a license from the vendor.
FL Studio is an established application and is perfectly safe to download from the official Image Line website. However, this is a heavily pirated program, and a number of illegally cracked versions exist on the Internet. These versions are unverified, and may contain malicious software. These versions should be avoided.
Though the difficulty of learning a new application is entirely subjective, it would be fair to say that it is not easy to learn. This application is a fully featured Digital Audio Workstation, and with any powerful piece of software such as this, there is a quite significant learning curve.
At this time, the app is not officially available on Mac. As per developers, Image Line, there are a couple of options. You can use Boot Camp to run Windows on your Mac and install the app there, or you can try the OS X Beta version of the app.
FL Studio does support VSTs, so Kontakt should work just fine. Native Instruments themselves have a tutorial on their website for installing their plugins with earlier versions of the app, as well as one for the latest version. Simply install Kontakt the way you would install any other VST plugin.
Though not as intuitive and simple as it would be to use with the intended Ableton Live, the Ableton Push controller can be made to work with this app. Some additional steps are required to get things up and running (tutorials are easy to come across), but it is doable.
Though well known EDM artist, Deadmau5, has been a user of this app in the past, it is widely though that he has since moved away from it, instead switching to Ableton Live. There have been claims that Deadmau5 has been involved with development of the app at times.
There are a number of options for the latest version of this app. The Fruity Edition comes in at €89. The Producer Edition, which is more feature rich than the Fruity Edition, is €189. The Signature Bundle comes with more plugins, and is €289. Finally, you can get the app and all plugins for €791.69
Due to the risk of accidentally downloading malicious software that could infect your computer, you should always download software from the official source. This app is available from the website of its developer, Image Line, and can be found in the "Downloads" section. You can download a trial or buy the full software.