Every geocache has a rating of difficulty and terrain, this is commonly shorted to D/T or D/T-rating. The difficulty and terrain is rated on a scale between 1 and 5 with the possibility of halves, for example the most common D/T-rating of all caches is 1.5/1.5. More information can be found in Geocaching.com's Help Center.
D/T in statistics
The different D/T-ratings can be sorted in a 9x9 grid, commonly referred to as a D/T grid, "D/T matrix" or "Fizzy grid". These can be found in many places across Project-GC, for more information see D/T grid.
Top #DT in one day is a top list for unique DT combinations in one day.
The general overview statistics shows Caches per DT for an entire country.