Unless your organization is especially small, you can’t get away with running SharePoint on a single server. Instead, you have to use multiple servers. When you use multiple servers to run an application, it’s called a server farm. You don’t have to start with a large farm because you can scale up by adding more servers as your needs require.

A server farm consists of multiple servers that work together to provide the SharePoint experience. Companies often find it necessary to span services across multiple servers to provide the performance and availability their organization requires.

Server farms are made up of physical servers that run logical services. The physical servers are the computer hardware that you can touch. The logical services are the applications that run on the physical servers. In many cases, your physical servers pull double and triple duty by running multiple logical services.

A minimum SharePoint installation usually has the following two physical servers:

• A front-end server that serves Web pages and other SharePoint services such as search and Excel Services.
• A back-end server where the database is stored. All the content displayed in SharePoint’s Web pages is stored in databases.

The distinction between physical servers and logical services is important. SharePoint requires a dozen or so logical services to run. That doesn’t mean you need a dozen servers.

Some of these services run on front-end servers, and others run on back-end servers. (A server’s designation as front-end or back-end is based on the logical services it provides. A back-end server provides some kind of service that the front-end server requires in order to function. A front-end server provides services directly to end users.)

Logical services are installed on one or more physical servers. The logical services running on a physical server determines the role of the server in the server farm. For example, the server running the database is often called the database server.

A database server is back-end because it provides data to front-end Web servers. Typically, an installation has at least two physical servers - an application server and a database server. The application server hosts all of SharePoint’s applications and serves Web pages to end users.

The number and kind of servers you deploy depend on many factors including the number of users, the volume of data, and your requirements for performance and availability. If you have lots of users accessing a relatively low volume of data, then you may decide to add Web servers.

Alternatively, if your deployment is data heavy, then you may deploy additional database servers. If you’re making extensive usage of Excel Services or InfoPath Forms Services, you may need to deploy separate application servers to host those services.