Below is a diagram of a (PCM) Powertrain Control Module. Vehicle manufacturer's create diagrams of the entire vehicle that can be complex and difficult to sort through, so they're broken down into system specific diagrams called block diagrams. Wiring diagrams show the wiring, connectors, and other system related information. They're like road maps for the wiring routed throughout a vehicle.
The diagram above has several different schematic symbols. Schematics are used in electronics; they contain standardized symbols to represent components in a circuit. The diagram above contains diodes, resistors, variable resistors, and other symbols representing solid state circuitry. Component A is a connector. Notice the black dots; they represent a splice or junction where wires are connected. When wires in a schematic cross without a dot, they are not connected.