In Explore I see both COUNT and D_COUNT as options for aggregation.
COUNT and D_COUNT (Distinct Count) are ways of aggregating data. Data aggregation is the process where a list of items is combined into one data point. For example, SUM is an aggregation where the listed values are added together.
COUNT counts the number of items being aggregated. D_COUNT counts the number of unique items there are being aggregated. Consider the following list.
Aggregated by COUNT on that list would give the result of 4 as there are four items. Aggregating that list by D_COUNT would give a result of 3 as two of the items are the same. The most common use for COUNT and D_COUNT in Explore is when aggregating ticket IDs. D_COUNT will make sure each ticket only shows up once, COUNT allows a ticket to be counted multiple times.
For more information on the different Explore aggregators see Changing metric aggregators.