Nowadays, most HF amateur operation is single sideband (see below). That means your radio will use either the upper sideband (USB) or the lower sideband (LSB) for a particular contact. You can legally use either on any frequency band. But by convention, some frequency bands use one; some use the other. Why?
Lower sideband (LSB)--The common single-sideband operating mode on the 40, 80 and 160-meter amateur bands.
Upper sideband (USB)--The common single-sideband operating mode on the 20, 17, 15, 12 and 10-meter HF amateur bands, and all the VHF and UHF bands.
[Source for both above: http://www.arrl.org/ham-radio-glossary]
It's a historical oddity. Early amateur SSB rigs used a 9 MHz IF system, and it was easier and cheaper to generate LSB below 9 MHz and USB above 9 MHz. With most designs these days, USB and LSB are equally easy to use, but we keep to the old convention.
You will notice that on 160, 80 and 40 meters, LSB is mostly used and on the higher frequency bands, USB is used. This is a leftover from the early days of SSB when radios were designed a certain way, when SSB was in its infancy. Operating habits will not likely change in the future.
Single Sideband (SSB)
SSB ("sideband", Single Sideband Suppressed Carrier) is the descendant of traditional AM. Compared to AM, SSB requires less than half the bandwidth and it offers much more "talk power", since the unnecessary carrier and second sideband are not transmitted. SSB requires more frequency stability and somewhat more complex circuitry than AM, but essentially all mainstream amateur gear for the HF bands now supports SSB voice.