The 2008 Nobel Prize in Physics has been awarded to Yoichiro Nambu "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics", and to Makoto Kobayashi and Toshihide Maskawa "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature".
This work included a description of how quarks can change flavor, predicted a third generation of quarks, and gave details of one way in which matter and antimatter are not exactly the opposites of each other.
Nambu developed a mathematical description of spontaneously broken symmetry in particle physics, not only for quarks. Kobayashi and Maskawa developed the concept for how CP violation (responsible for some of the difference between how matter and antimatter act) could occur in the weak force, and how that is reflected in the interactions of quarks.
The work of Kobayashi and Maskawa is codified in what is known as the CKM Matrix (also named for Nicola Cabibbo). Kobayashi and Maskawa extended previous work by Cabibbo and proposed the three generations of quarks that are now known to exist.
Experimental determination of CP violation has been shown in various experiments such as that in kaons in 1964 (leading to the Physics Nobel for Val Fitch and James Cronin in 1980). More recent experiments done at the B factories--the BaBar detector at Stanford Linear Accelerator Center and the Belle detector at KEK in Japan--demonstrated an amount of CP violation just as predicted by Kobayashi and Maskawa.
It was really the predictions by Kobayashi and Maskawa that opened the possibility of seeing CP violation in B mesons and that led to the creation of the BaBar and Belle experiments.
The concept of spontaneous symmetry breaking was first introduced to particle physics by Yoichiro Nambu who studied the phenomenon in superconductivity. He translated those ideas to particle physics and ushered in the construction of the Standard Model of particle physics. The Higgs boson is thought to be responsible for particles acquiring mass, and that process is also an example of symmetry breaking.
The work recognized by the 2008 Nobel Prize in Physics is a key step in the long development of physicists' understanding of the fundamental laws that govern the particles and their interactions.
Relevant articles in symmetry magazine:
Deconstruction: CKM matrix/Unitarity triangle
BaBar's window on the weak force
explain it in 60 seconds: B factories
explain it in 60 seconds: The Standard Model
explain it in 60 seconds: CP violation
explain it in 60 seconds: antimatter
Other relevant Nobel Prizes in Physics:
2004: Gross, Politzer, Wilczek--asymptotic freedom in the strong force
1999: 't Hooft, Veltman--electroweak force
1990: Friedman, Kendall, Taylor--quark model
1980: Cronin, Fitch--symmetry breaking (CP violation) in K mesons
1979: Glashow, Salam, Weinberg--electroweak theory
1969: Gell-Mann--classification of elementary particles
1965: Tomonaga, Schwinger, Feynman--quantum electrodynamics
1957: Yang, Lee--parity violation