Several agents are used to perform the actions necessary to move the replicated data
from the publisher to the distributor and finally to the subscriber:
* Snapshot Agent,
The Snapshot Agent is used for creating and propagating the snapshots
from the publisher to the distributor (or snapshot location).
The Snapshot Agent creates the replication data (the snapshot)
and creates the information that is used by the Distribution Agent
to propagate that data (the metadata).
The Snapshot Agent stores the snapshot on the distributor (or anywhere that you specify).
The Snapshot Agent is also responsible for maintaining information
about the synchronization status of the replication objects;
this information is stored in the distribution database.
* Log Reader Agent,
The Log Reader Agent is used in transactional replication
to extract change information from the transaction log on the publisher
in order to replicate these commands into the distribution database.
Each database that uses transactional replication has its own Log Reader Agent on the publisher.
* Distribution Agent,
The Distribution Agent propagates snapshots
and transactions from the distribution database to subscribers.
Each publication has its own Distribution Agent.
If you are using a push subscription, the Distribution Agent runs on the distributor.
If you are using a pull subscription, the Distribution Agent runs on the subscriber.
* Merge Agent,
The Merge Agent is used in merge replication
to reconcile (merge) incremental changes that have occurred since the last reconciliation.
When you use merge replication, the Distribution Agent and the Snapshot Agent aren't used
- the Merge Agent communicates with both the publisher and the distributor.
* Queue Reader Agent.
The Queue Reader Agent is used to propagate changes made to subscribers of snapshot
or transaction replication that have been configured with the queued updating option.
This option allows changes to be made on the subscriber without the need to use a distributed transaction.
Source: Microsoft SQL Server 2000 Administrator's Companion Book