Ski touring and backcountry skiing are both extremely similar. Both require avalanche safety equipment and take place outside resort boundaries. Ski touring is generally referring to going on an extended tour off-piste, away from a ski resort. A ski tour can last an extended period of time from a day, to a few weeks. A ski tour does not utilize lifts or any other forms of transportation other than skis.
Backcountry skiing generally refers to off-piste skiing in unmarked or unmanaged terrain. Backcountry skiing can be accessed via lift, snowmobile, or Snowcat as well as skinning or hiking. The term slack-country can also be used to describe skiing at a resort, outside resort boundaries.