The Intel Core i7 processor has a feature known as "hyper-threading." Quoting their website:
Intel® Hyper-Threading Technology (Intel® HT Technology)1 uses processor resources more efficiently, enabling multiple threads to run on each core. As a performance feature, it also increases processor throughput, improving overall performance on threaded software.
Of course, this sounds a bit like an advertisement, and it doesn't show whether it has any less desirable qualities to it, such as increased power usage.
What exactly is hyper-threading? When should I buy a processor that utilizes this?