An electrical circuit is a closed loop interconnection of devices (which we call elements). Voltage and current values are continuous-time signals, so we often call these analogue circuits. The practice of analysing and solving for voltage/current values is circuit analysis.
Elements include linear and non-linear elements. Linear circuits only contain voltage/current sources, resistors, capacitors, and inductors; they are key examples of linear time-invariant systems. Electronic circuits expand the ability of linear circuits, by containing non-linear, active, often semiconductor-based elements.
Basics
Interconnects between elements are wires. We assume for wires to be ideal (with zero resistance), and for all circuit elements to be ideal. Two main parts of circuits are nodes, and branches. Nodes are junctions between two or more devices. Loops are closed paths through an ordered sequence of nodes, with no duplicate nodes.
The basic equations of resistors, capacitors, and inductors, are linear.