A Detail Fact table is normally a transactional table that represents the business data at its lowest level of granularity. In many ways, these tables reflect the business processes within the organization. These Fact tables are usually large and are focused on a specified analysis area.
There may be quite a large number of Detail Fact tables in a data warehouse implementation, of which only a few are used on any regular basis by the end user community. The disadvantage of these Fact tables is that they provide isolated pools of information. Although joined by conformed dimensions, it is still often difficult to answer queries across the various analysis areas. They do however provide the ultimate drill down for all information, and also the platform on which to build higher level and more complex Fact tables.
In terms of the time dimension, Detail Fact tables are typically at a daily or even hourly granular level.
An example of a Detail Fact table may be the sales, or orders Fact tables that have a daily granularity, and show all sales by product, by customer etc. on a given day.