SQL Server 2005 provides three main types of replication: snapshot, transactional, and Merge.
Publications and Articles
Articles: database object
Publication: SQL Server group’s articles from the same database in a unit called a publication.
Server Roles
You can configure a server as Publisher, Distributor, and/or Subscriber.

• The Publisher is the original owner of the information that is published.
• A Publisher is the only place where data can be modified.
  

 

The Distributor is responsible for managing the distribution database which stores replication status data, metadata, and, in some replication scenarios, the actual data that SQL Server replicates

A single database server instance can act as both the Publisher and the Distributor.

The server that then receives copies of the publication and provides the data to end users and applications is called the Subscriber.

Subscribers have a read-only copy of the database; in other configurations, you can update information in the Subscribers and replicate those changes back to the Publisher.

Push and Pull Subscriptions

With a push subscription, the Distributor copies the data to the Subscriber database. With a pull subscription, the Subscriber retrieves the data from the Distributor.

When your communication infrastructure has stable and permanent connections among replication servers, a push subscription offers the advantage of providing a central management point to all replication agents that manage the replication process. This results in less administrative overhead and easier troubleshooting procedures.

However, when the replication configuration includes many Subscribers, the distribution process for push subscriptions can tax the hardware resources of a Distributor.

Furthermore, servers or clients that connect on demand are better configured as pull subscriptions.

Replication Types

Snapshot replication the easiest replication type to understand because it is conceptually similar to a full backup and restore.

 

Transactional replication

It makes an initial complete copy of the data, and then all subsequent copies transfer modified data only.

Transactional replication uses the transaction log to apply to the destination data the same transactions performed on the source data. Because the same modifications are applied at both ends, the information is identical at the Publisher and the Subscriber.
This replication type is frequently used for transactional tables, such as an Order Details table in a retail database.

 

Merge replication

When your environment requires the ability to support simultaneous data modifications in the Publisher and Subscriber databases, merge replication offers a solution.

 

Replication Agents

SQL Server uses a group of programs called replication agents to execute the replication process.

To using SQL Server by many department to manage your business you using Use Multiple Instances of SQL Server 2005

SQL Server 2005 supports the capability to install multiple instances (or copies) of SQL Server 2005 or to install SQL Server 2005 alongside earlier versions of SQL Server on the same server. During the installation process, the DBA can choose to install an instance without a name—in which case, the instance name will take the name of the server—as a default instance. To install multiple instances of SQL Server besides the default instance on the same computer, the DBA must give the additional instances different names. Being able to install multiple instances of SQL Server lets you have system and user databases that are independent of each other. This capability not only lets you work with earlier versions of SQL Server already installed on your Computer but also lets you test development software and operate instances of SQL Server 2005 independently of each other.

SQL Server 2005 can support up to 50 instances when it’s not clustered, When SQL Server is clustered, and the maximum number of instances is 25

As to the maximum recommended number of instances, that is going to depend on the resources available on your server and the resources that each instance requires.