It is a fact table which does not contain any measurable data.

EX: Student attendance fact (it contains only Boolean
values, whether student attended class or not ? Yes or No.)

A Factless fact table contains only the keys but there is 
no measures or in other way we can say that it contains no 
facts.


 Generally it is used to intigrate the fact tables.

factLess fact table def as mentioned above..
Where it is used?
These are used particularly to resolve many to many 
relationships,which will create duplicate counts in OLAP 
Cube creation.
Or it can be used to avoid loops which will decrease the 
query performnce while reporting

Factless fact tableis used for the  converting the star 
schma into snowflake schema

Factless fact table contains only foreign keys. We can have two kinds of aggregate functions from the factless fact one is count and other is distinct count.

2 purposes of factless fact

1. Coverage : to indicate what did NOT happen. Like to 
answer questions like which product did not sell well in a 
particular region.

2. Event tracking: To know if the event took place or not. 
Like: Fact for tracking students attendence will not 
contain any measures.