Active Directory uses pull replication. In pull replication, a destination replica requests information from a source replica. The request specifies the information that the destination needs, based on its knowledge of changes already received from the source and from all other domain controllers in the domain. When the destination receives information from the source, it applies that information, bringing itself more up-to-date. The destination's next request to the source excludes the information that has already been received and applied.
The alternative is push replication. In push replication, a source sends information to a destination unsolicited, in an attempt to bring the destination more up-to-date. Push replication is problematical because it is difficult for the source to know what information the destination needs. Perhaps the destination has received the same information from another source. If a source sends information to a destination, there is no guarantee that the destination is going to apply it; if the source assumes otherwise, the system is unreliable.