Each system will have inputs and output. Example of an input can be battery which is connected to a circuit. The output is some entity which we want to measure in a circuit element.
In the example below, the input is the battery supplying voltage . Since, we are interested in current flowing in the resistor, the output is current .
The process of predicting the output for given inputs in a electronic system is circuit analysis. The inputs can be of various forms e.g, mic converts audio to electrical signal. Alternatively, the output can also be a non-electrical entity e.g., speaker converting the electrical signal to audio. In general the electronic system will have multiple inputs (can be signal inputs or power sources) and multiple outputs.
We need to understand the basic elements to do circuit analysis.