In general the definition of LDAP, Active Directory and SSO are as below:
LDAP (Lightweight Directory Access Protocol) is a software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network, whether on the public Internet or on a corporate intranet.
Lightweight Directory Access Protocol - Wikipedia
Active Directory (AD) is a directory service that Microsoft developed for Windows domain networks. It is included in most Windows Server operating systems as a set of processes and services. Initially, Active Directory was only in charge of centralized domain management.
Active Directory - Wikipedia
Single sign-on (SSO) is a session and user authentication service that permits a user to use one set of login credentials (e.g., name and password) to access multiple applications.
Single sign-on - Wikipedia
Now if you want to know how to integrate them with Service Now then follow the Links provided by Pradeep. Besides that links, below links are for sso integration.
http://wiki.servicenow.com/?title=External_Authentication_(Single_Sign-On_-_SSO)#gsc.tab=0
http://wiki.servicenow.com/index.php?title=Multiple_Provider_Single_Sign-On#gsc.tab=0
