Dual core= 2 proccessor cores in a single package. It's like having dual cpu's, but they go in a single socket. With certain Xeons and Opteron dual cores, you can have dual dual cores, giving you a total of 4 cpu cores in a single machine.
hyperthreading and hypertransport have nothing to do with eachother.
Hyperthreading is a form of SMT-simultanios multple threads. It basicly make the operating system think there is 2 cpu's when there is actualy only one. It can give ~20% performance increase for programs written to take advanatage of multiple CPU's.
Hypertransport is the communication bus used by A64/Opteron as a replacement for the front side bus. It has a much higher bandwidth than the normal Front side bus that is used by Intel CPU's or Athlon XP and old AMD cpu's.