What is an Electric circuit?
An Electric circuit is a path along which electric current flows.
In the below diagram, all the terminals of the electric cell and bulb are connected, and a closed loop is formed. This closed loop is an Electric circuit.
What is Electric current?
Electric circuit
 Electric current is generally defined as the rate of flow of electrons through a conductor. It also means the flow of electrical energy through a circuit.
How does electricity flow through a circuit?
Let us consider the same circuit consisting of a cell, copper wire and a bulb.
The negative terminal of the cell repels the electrons in the copper wire near it. When these electrons drift away from the terminal, they, in turn, push their neighbouring electrons. In this process, these electrons transfer the energy to their neighbouring electrons. This process of energy transfer continues and in this way, the energy flows throughout the circuit.
Electricity flowing through a circuit
 How does electricity flow through a bulb?
The electrons in the copper wire transfer the electrical energy to the electrons in the metal rod connected to one of the terminals of the bulb. This energy then flows through the rod, then through the filament and finally out from the second terminal. In this way, the energy flows through the bulb.
When the energy flows between the two ends of the filament of the bulb, it heats the filament which in turn glows and gives off light. The energy flows as long as the circuit is connected or until the cell drains out.
Electricity flowing through the bulb
  If we interchange the wires connected to the two terminals of the bulb, the circuit is still complete and the bulb still glows. However, if we connect both the wires from the battery to only one terminal, the bulb will not glow. This is because the bulb no longer remains part of the circuit.
Bulb does not glow
 In the above figure, the positive and negative terminals of the battery are connected to only one terminal of the bulb. So, the energy is flowing from the negative terminal directly to the positive terminal, without flowing through the bulb. And hence the bulb does not glow.
But this could result in the battery exploding. We should therefore never connect the positive and negative terminals directly. This heats up the wire quickly and drains the battery really fast.