Active Directory Federation Services (ADFS) allows the sharing of identity authentication between two trusted partners beyond the boundaries of their respective Active Directory (AD) forests. This feature is available within Windows servers and provides users with Single Sign-On access to systems and applications across organizational boundaries.
ADFS is becoming very popular due to the dramatic shift of enterprise customers towards cloud-based services such as Microsoft Office 365. ADFS enables enterprise users to utilize existing, local, AD credentials to access Office 365 or other trusted networks through an AD federation. The benefit to users is that only a single credential is needed to access applications. The single access credential works as long as the trusted relationship between the AD forests remains intact.