If I had written this answer 6 months ago, I would have said with some conviction that they SEEM to be fundamental particles with no known internal structure that has yet been detected. Then, in the March 7, 1996 issue of Nature on page 19, it was announced at Fermilab by a team of 450 physicists that their scattering experiments had shown that something not quite expected had occurred. They detected a slight increase in the number of 'jet' events that the so-called Standard Model had predicted for the interactions between colliding beams of quarks and gluons. Among the possible explanations, assuming that the data hold up in future experiments, is that the quark may have internal structure which means that the Standard Model has to be severely modified. A competing theory of the strong interaction called 'technicolor' has for a decade proposed that there are other particles even smaller that quarks, and a new force even stronger than the strong force to bind them together into quarks and electrons. Another explanation for this anomaly is that some new massive particle was temporarily produced, perhaps one of the missing 'supersymmetry' particles which extensions of the Standard Model have been predicting for over 15 years. Stay tuned as more data comes in!!