SOPC Builder (System on a Programmable Chip Builder) is software made by Altera that automates connecting soft-hardware components to create a complete computer system that runs on any of its various FPGA chips. SOPC Builder incorporates a library of pre-made components (including the flagship Nios II soft processor, memory controllers, interfaces, and peripherals) and an interface for incorporating custom ones. Interconnections are made though the Avalon bus. Bus arbitration, bus width matching, and even clock domain crossing are all handled automatically when SOPC Builder generates the system. A GUI is the only thing used to configure the soft-hardware components (which often have many options) and to specify the bus topology.
The resulting "virtual" system can then be connected to the outside world via the FPGA's programmable pins or connected internally to other soft compoments. The FPGA's pins are routed to connectors, such as for PCI or DDR, or -- as is often the case in embedded systems -- to other chips mounted on the same PCB.
Resource utilization on an FPGA hosting an SOPC Builder system is very modest by modern standards. FPGA devices supporting SOPC systems include almost all Altera FPGAs (and even some CPLDs) ranging from $5 to $5,000 in price.