The main purposes of a PCB are:
1. Interconnect all components electrically
2. Provide mechanical support to the components mounted
3. To provide the conduit for electricity and to help dissipate heat
At this point it might be important to make a distintion between boards and packages. The first PCB type are normally associated with large form factor boards where a number of components are attached to, the most common example of these being the motherboard found in all PCs.
On the other hand, packages are smaller boards typically used as interposers or substrates for integrated circuits (IC), normally built in individual pieces of Si or chips. They help spread out the signals and provide a conduit for power to the IC. In a given PCB a number of packages can be assembled or mounted in addition to capacitors, inductors, resistors, sockets, connectors, etc. with the PCB providing the connections for those components to interact.
' A Gerber File is a standard file format used by printed circuit board (PCB) fabrication houses that contains information necessary for computer controlled machines to draw exact patterns for circuit boards. These patterns are typically used to assemble and electrically connect electronics assemblies. The patterns usually contain features such as land patterns, signal traces, drilled holes, milling and cutting information. The Gerber Format is originally a subset of EIA RS-274-D. Later the format was then extended by the so called "Mass Parameters" and was then also called RS-274-X. ' from Wikipedia Gerber File
For more discussion about Gerber and its application to PCB manufacture click here