Block diagrams are used to understand (and design) complete circuits by breaking them down into smaller sections or blocks. Each block performs a particular function and the block diagram shows how they are connected together. No attempt is made to show the components used within a block, only the inputs and outputs are shown. This way of looking at circuits is called the systems approach.

Power supply (or battery) connections are usually not shown on block diagrams.

source: http://www.kpsec.freeuk.com/study.htm

This is the block diagram of the most of regulated power supply.

Notes:

• Transformer – steps down 230V AC mains to low voltage AC.
• Rectifier – converts AC to DC, but the DC output is varying.
• Smoothing – smooth the DC from varying greatly to a small ripple.
• Regulator – eliminates ripple by setting DC output to a fixed voltage.

The following is a general block diagram of a radio receiver:

Notes:

• Aerial – picks up radio signals from many stations.
• Tuner – selects the signal from just one radio station.
• Detector – extracts the audio signal carried by the radio signal.
• Audio Amplifier – increases the strength (power) of the audio signal. This could be broken down into the blocks like the Audio Amplifier System shown above.
• Loudspeaker – a transducer which converts the audio signal to sound.

This is the block diagram of an completed audio amplifier from input (microphone for example) to the output (loud speaker). You may add some additional block like an equalizer between pre-amplifier and tone control, or add a crossover between power amplifier anf loudspeaker.

Notes:

• Microphone – a transducer which converts sound to voltage.
• Pre-Amplifier – amplifies the small audio signal (voltage) from the microphone.
• Tone and Volume Controls – adjust the nature of the audio signal. The tone control adjusts the balance of high and low frequencies. The volume control adjusts the strength of the signal.
• Power Amplifier – increases the strength (power) of the audio signal.
• Loudspeaker – a transducer which converts the audio signal to sound.

Circuit diagrams show how electronic components are connected together. Each component is represented by a symbol and a few are shown here:

Circuit diagrams and component layouts
Circuit diagrams show the connections as clearly as possible with all wires drawn neatly as straight lines. The actual layout of the components is usually quite different from the circuit diagram and this can be confusing for the beginner. The secret is to concentrate on the connections, not the actual positions of components.