Segmented markets vs. term structure
The segmented markets theory was introduced by American economist John Mathew Culbertson in a 1957 paper called The Term Structure of Interest Rates. In this paper, which is basically arguing against the idea of term structure, Culbertson argues against Irving Fisher's idea of term structure -- essentially the opposite of the segmented markets theory.
In the term structure of interest rates theory, bond interest rates are related to each other, even if it's not a terribly obvious relationship. The theory is still widely used to help understand how the economy is performing and may perform in the future.
How does the yield curve fit into this?
The yield curve comes from plotting the differences between yields for both short-term and long-term bonds. It typically compares the yields of bonds with equal credit ratings but different maturity dates, including three-month, two-year, five-year, and 30-year U.S. Treasury bonds.
Based on the shape of the yield curve, economists can make several assumptions about the current economy and even potentially predict what it may do in the near future, using various bond interest rate theories, including the market segmentation theory.