The Tri-Cities is the combination of Kingsport, Johnson City, and Bristol, TN/VA
Kingsport is largly known for Eastman/Kodak which is by far the largest employer in the area (and in east TN). Johnson city has the campus of ETSU, and Bristol (the most pleasent of the three) is somwhat famous for is Nascar motor speedway. I'm not sure what the metro population is for this area, but I have heard is either 5th or 6th in the state in terms of size.
Regarding Knoxville, there are a lot of exciting projects going on here, and the speed they are building office parks and neighborhoods in astounding. I am planning on doing a photo study of downtown renovations as well as the new Turkey Creek development in West Knoxville (a pretty incredible development)