What differences between iWatch and smart watch?

I only know the difference is iWatch is the Apple brand smart watch. Also I noticed Apple iWatch is way more expensive than any other smart watches, why is that?

Will any kind/brand smart watch work with iPhone?

What are differences between these smart watch brands: Pebble, Microsoft, Samsung, & Apple iWatch?

Which brand/kind smart watch do you recommend and why?

The same reason why All Apple products are way more expensive than others: because people will pay it. No other reason.

Apple has better designs. Thats the same reason why iphones never lag. People can compare specs all they want, but iphones don't lag and other phones always do. The watch will be the same way, it will just be better especially as more apps appear for it.

Its called the Apple Watch for the record. Not iWatch.
The Apple Watch is not more expensive than most other flagship smartwatches. If you are comparing it to 1st generation models like the LG G Watch then yes it would be of course.

Select Android Wear smartwatches would work with it, as well as ones from Pebble

Pebble make their own smartwatches that run their own OS

Microsoft doesn't make a smartwatch as such. More a smartband, the latest being the Band 2

Samsung makes several which run 3 different OSs (sort of a result of their 'lets be first' strategy. Android, Android Wear & their own Tizen. The latest being the Samsung Gear S2 which runs Tizen

Apple of course makes their own smartwatch which runs watchOS, based off of iOS & come in 3 model options. The Sport, standard & Edition.