hi bro's since we are on the flash topic..I've seen flash like this around, third party.,..how good is it compared to those from canons??

http://cgi.ebay.com/Vivitar-283-External-A...1QQcmdZViewItem
http://cgi.ebay.com/Nissin-Di481-Dedicated...1QQcmdZViewItem

vivitar and nissin?? not sure la./...anyway how should i determine a flash?? what do they mean when they provide this specs??

Key Features
Dedication -Dedicated
Guide Number -36 @ ISO 100 (85mm.)
Type -Regular Flash
Zoom Range -24 - 85 mm. (they have zoom on flash?? how la how it work?? what if i have a 17mm kit lens? meaning cannot use this flash??)
Bounce -With Bounce (bounce wat? can someone enlighten me? is it the "rotatable" means can bounce??)
Power Source -4 x AA Batteries
TTL Metering -With TTL Metering (this for low light rite?)
Weight 9.2 oz.

someone selling the tamron 17-50mm f2.8, just wanna know how "bokeh" is this lens a??