According to Tizen.org:
Tizen is an open source, standards-based software platform supported by leading mobile operators, device manufacturers, and silicon suppliers for multiple device categories, including smartphones, tablets, netbooks, in-vehicle infotainment devices, smart TVs, and more.
Tizen is actually the product of a previous merger between Samsung's Linux Project and the MeeGo platform, previously sponsored by Nokia. With the former Finnish phone giant switching it's attention to the Windows Phone 7, MeeGo was anxious to find a new backer.
According to bada.com:
Samsung bada is a smartphone platform , which is created for wide range of device...bada is a new blue ocean of mobile applications for developers, and customers will have a wider choice of smartphones with cost-effective yet powerful bada-powered phones.
bada (Korean for ocean) has been Samsung's primary platform for almost 10 years and has seen features added such as, "multipoint-touch, 3D graphics, an enhanced UI, and of course, application downloads and installations."
With the integration of Tizen and bada, Samsung is hoping to shift the focus of the bada development community towards Tizen and facilitate a reduction in the number of mobile operating systems Samsung uses. But look for bada to stick around, as Kang suggested Samsung may continue to use bada on low-powered devices, and also noted that it will take time to develop Tizen as Samsung's primary platform.