A circuit is simply an electrical path in which current flows. It can be as simple as a battery lighting a light bulb or as complex as some of the circuits in the computer you're using. A circuit may contain any number of devices as long as current flows into one terminal and out of another terminal back to the power source, it's a circuit. If you'd ask most people how often they make or break the connections in a circuit, they'd likely say rarely or never. What they don't realize is that every time they flip a lightswitch on or off, they're making or breaking a circuit.

Note:
Some people say that a bad (intermittant) connection is a short (circuit). A short circuit would cause current to flow through a path other than the intended path. Usually the short path has very little resistance, causing excessive amounts of current to flow out of the power supply. When a power wire shorts to ground causing a fuse to blow, that's a short circuit. If something turns on and off when you move a wire or connector, that's an intermittantly open connection (open when the device turns off).

Conventional Flow vs Electron Flow in DC Circuits:
In the DC circuit demo above, the current flow is 'electron' flow. This means that it shows the 'electrons' (note the little E in the moving particle) flowing from the negative terminal of the power source (the battery). This is the way the current is actually flowing. Most of the time, people consider the flow of current to go from the positive terminal to the negative terminal. This is called 'conventional' current flow. If you need to imagine the current flow from positive to negative (much easier for most people), that's fine. When you hear someone arguing about current flow and it's actual direction of flow, ask them if they're talking about electron flow or conventional flow.

As a side note... In AC circuits, the polarity is continuously changing so you have electrons flowing both ways in the circuit. Keep reading through the directory in order and the AC/DC concept will be explained shortly.

Note:
Unless otherwise noted (by the use of an 'E' in the moving particle or by a text note), current flow will be conventional flow. In some demos, there is a moving particle which indicates current flow but it has an 'O' in it instead of an 'E'. This means that the particle is not an electron and it shows conventional flow.