1. multi-out is what some virtual instrument can use. You can have one instrument that outputs its sounds to multiple channels. i.e. a sampler instrument like Kontakt can load a bass, guitar, drums, all in one instance of the plug-in and route the output to different virtual channels so you can mix them seperately.
2. The full audio track and ASIO in are indeed for Producer and XXL editions. Full audio tracks means you can see the actual wave files in your playlist (see here: http://flstudio.image-line.com/help/html/playlist_audio.htm ). ASIO in means you can record audio with an ASIO capable soundcard or by using the free software ASIO4ALL. Note, the Producer and XXL versions also come with Edison, a great audio editor.
3. That's where you can enter individual notes, to make melodies and chords. It's very advanced and one of the best around. Check here: http://flstudio.image-line.com/help/html/pianoroll.htm
4. Automation means when you record something you do to an instrument, effect, etc. For instance, when you record and you fade out a sound, that event can be recorded in the event automation. Very useful, you'll want this the more you learn about FL Studio.
5. The Producer and XXL version include a version of Synthmaker, which allows you to build synths. The XXL comes with a great synth called Sytrus (it's also in the demo version). Otherwise, don't buy FL Studio for the included sounds only. There are many, many freeware instruments available for download online (feel free to email me when you help where to get them)
6. Garageband is much more simple really. FL Studio can be daunting at first but once you learn the basics and start to get into more advanced things (check YouTube for tutorials) you will find it's incredibly capable and has many advanced features you'd normally only find on much more expensive software.
I've been using the Producer Edition for years now, my favorite audio software. The XXL version only makes sense if you really like Sytrus because it's much more if you'd buy it seperately. XXL also includes some other nice instruments that aren't in the cheaper versions.
In case you end up buying FL Studio, here's a link for 10% discount on it: https://support.image-line.com/jshop/shop.php?offer=Default&promo=BACJBAB469
Be sure to get all you want in one go (i.e. if you'd like to get one of image-line great synths like Morphine, Poizone, Toxic, etc) as this link only works for new customers.