SSRS Farm Overview
Building a SSRS farm will require Enterprise or Business Intelligence editions of SQL Server for versions 2012 and 2014. SQL Server allows for the deployment of 2 or more servers running SQL Server Reporting Services to increase the performance of your reporting environment.
SSRS stores its data in 2 databases (reportserver and reportservertempdb). When adding another server to the farm it will leverage the same databases as the original installation. This allows the servers to process the same catalog of reports (rdl files) and increase server processing capacity. Each server in the farm can be accessed via its own URL that enables you to place reports on specific servers such as adhoc reports on server 1 and scheduled reports on server 2. Another option includes using a Network Load Balanced (NLB) cluster to create a shared URL that will spread out the incoming report requests among all the servers in the farm.
If you are noticing performance issues in your stand alone SSRS environment such as longer than normal execution times or users having trouble accessing the reports. Then adding an additional server to your installation may help. The SQL Customer Advisory Team (SQLCAT) has a great article on performance and optimization referenced below.
SQLCAT Performance Optimization for SSRS:
MSDN contains many articles on setting up the farm: