Can someone tell me about P4 HT (HYPER THREADING) ................?

Can someone tell me what is p4 HT?? and what it's use?? and it will harm the computer hardware or processor when it is enabled???And why it is used.And how to enable or disable it.I have a CPU DELL optiplex gx620 with 3.2 GHz processor and 1gb ram what should i call it P4 HT or Pentium D?? Please give the answers of my questions cuz i really wanted to know about it.