Q. What is Azure AD B2B?
A. Normally to grant users access to a service authenticated with Azure AD that user requires an Azure AD account in the Azure AD tenant. Today many organizations leverage Azure AD and have their own Azure AD tenants. With Azure B2B (Business to Business) it will be possible to create cross-company relationships enabling users from other Azure AD tenants to be granted access your resources. This enables your business partners to use their own local Azure AD accounts to access resources and avoid you having to maintain identities for users outside of your organization.
This is enabled through a simple CSV file of the users in the external Azure AD tenant which is imported to your Azure AD and those users will be sent invites and then be able to be shared access to your resources. Microsoft has a step-by-step available at https://azure.microsoft.com/en-us/documentation/articles/active-directory-b2b-collaboration-overview/ and at http://blogs.technet.com/b/ad/archive/2015/09/15/learn-all-about-the-azure-ad-b2b-collaboration-preview.aspx.