As it relates to SQL Server, replication is a way of keeping data synchronized in multiple databases. Implementing and maintaining replication might not be a simple proposition: If you have numerous database servers that need to be involved in various types of replication, a simple task can quickly become complex. Implementing replication can also be complicated by the application architecture.
Microsoft SQL Server has supported replication since version 6.0, and setting up replication has become significantly easier over the years (in fact, 99 percent of replication setup can be accomplished by clicking through replication wizards). However, replication involves much more than setup, and unfortunately there aren't many sources of information for implementing and troubleshooting it. The only way to learn replication is to dig through the knowledge base articles and figure things out on your own.
The article continues at http://www.informit.com/articles/article.asp?p=169612