There are three models of Replication supported by SQL Server 2008. Transaction Replication, Merge Replication and Snapshot Replication.
Snapshot Replication This is most simplest model of Replication. Snapshot Replication just restores the initial set of data and does not support any incremental restore of logs.
Agent: Snapshot Agent, Distribution Agent
Transactional Replication This is most common replication model. Typically this Model starts with initial snapshot. Using this model, Log Reader service reads the transactions and send them over to Distribution Server. Distribution server then distributes the transactions to all subscribers. This entire process by default runs continuously, however this can be scheduled to run on specific intervals.
Agents: Snapshot Agent, Log Reader Agent, Distribution Agent.
Merge Replication requires UniqueIdentifier type column in each published table. If this column type is not already added, it adds this column. To track changes, Merge Replication also adds Insert, Delete and Update Triggers for each of the articles (aka Published Objects). These triggers use the UniqueIdentifier type column in each table to identify the changes, which was added above.
Agents: Snapshot Agent, Merge Agent.
There can either be Push Subscription or Pull Subscription. In case of Push Subscription, Distribution Agent and Merge Agent runs at the Distributor. However, in case of Pull Subscription, Distribution Agent and Merge Agent runs at Subscriber.