Federated identity management is a way to connect identity management systems to each other, and single sign-on is a feature of this capability.
Federated identity management gives you single sign-on, but single sign-on does not give you federated identity management.
Here's some insight from Irv Kantar, business manager at ID Federation, which developed SignOn Once as the single sign-on process for the insurance industry:
"Federation is based on trust among members where one or more organizations agree to trust identities created by another organization, as long as all members follow the same rules. E
ach organization is then 'federated' with the other participating members.
For ID Federation, these rules and standards are contained in our trust framework."
Get more information and see the trust framework at idfederation.org.