What happens to generated electricity if nobody uses it? Let's take for example hydroelectricity produced at a dam and not consumed by any household or industry... Will the electricity need to be stored? What will happen if it is not stored? Does it flow like water and get wasted back to the earth? What happens to it?
 A: It is not practical to store the actual electricity. It can be stored, for example, in a battery as chemical energy, and then recovered at a later date as electrical energy. But this expensive and, in general, the electrical output power of a hydroelectric plant will be adjusted to closely match the load requirements. This can be done by adjusting the water flow through the turbine or by controlling the electrical operation of the generator.  For example, if the generator windings are not energized, the turbine blades will just spin freely in the water without generating power and without significantly impeding the water flow.
A: In the case of a dam with several turbines, it is possible to close some of them if there is not enough comsumption. In this case the energy is stored by increasing the volume of the water reservoir. There is always periods of drought, and water storage is like a battery.
Another possibility in the raining season, if the dam's reservoir are full, is to shut down thermal plants if there is no comsumption for all hydro+thermal. Of course the country system must be smart connected by transmission lines to be possible such changes.
A: If nothing is attached to the power plant, no electrons flow out of it. As the plant is no battery, with a material barrier between the electrons to stop them flowing back and neutralize the potential difference, you have to maintain the voltage, which costs money. Sometimes (for whatever reason) the maintaining of the potential costs more than what the electricity planters get from the users. Which is why sometimes customers are asked to reduce that cost for you by giving them money if they let fans shoot some breeze or washing machines clean some clothes (which will reduce the power to maintain the needed potential: supplying customers with flowing electrons actually costs less than maintaining the potential without anything attached; it would be best for the plant when customers use the electrons out of themselves of course, because then the plant needs less power and gets money on top! Which is not to say that the more customers use, the less power is needed; there is an optimum). You could even listen to music and get money for it! This reduces the costly power needed to maintain the potential. So the net result is cost reduction (if you don't pay the users too much, of which I'm sure they don't though).
You could reduce the potential, by using the power of the electrons to invest in static potential or by shooting the potent electrons in the Earth or whatever. Costs money both. Or you could reduce the force for maintaining the electrons at full potential.
So, let them flow freely, customers being a means "to Earth" (for which they are paid, or at reduced rates at certain hours), as there are no metal wires connected to Earth, use them to create other forms of (static) energy, or just reduce the generated force needed to maintain the potential of the electrons.
If, on the other hand, too many are connected,
A: In cases like this, it's helpful to think of electricity as flowing like water.* What happens if the pumps at a municipal water station pump more water than the consumers use?
Well, as the pumps (generators) try to shove more water (electrical energy) into the pipes (wires), the pressure (voltage) of the whole system will increase. This pressure (voltage) makes it harder for the pumps (generators) to push water (electrical energy) into the pipes (wires), slowing them down. In addition, the higher voltage makes the water (electrical energy) flow out of the taps (electrical devices) faster, so each home will naturally consume more water (electrical energy).
*Yes, there are flaws in the water analogy, but for a basic overview, it works perfectly fine.
A: In the power system, electricity must be consumed at the moment when it is produced. Otherwise, voltage and frequency deviations occur which could lead to a power outage. Imagine the transmission line is a capacitor: the voltage on the line is then defined as
$$v = \frac{1}{C} \int_{0}^{t} \bigl( i_\text{prod}(\tau) - i_\text{cons}(\tau) \bigr) d\tau$$
where $i_\text{prod}$ denotes the power produced by power plants, and $i_\text{cons}$ denotes the power consumed by consumers. Note that storage banks can be both producers and consumers, depending on what you want to do with the power. This equation is an oversimplification of what actually happens on the power lines, but it provides a rough idea.
There are several layers of planning of production and storage of electricity in the power system. Power system operators put a lot of effort into planning how much electricity will they produce, which of course depends on the predicted consumption. This is why renewables represent a problem (disturbance) for the grid - their power output is difficult to accurately predict as it strongly depends on atmospheric conditions. Large storage facilities are needed to counter the intermittent behavior of the renewables, i.e. to help stabilize operation of the power system.
Different types of power plants include: (i) Thermal, which produce steam which turns rotor blades which produce electricity on the stator. Steam is produced by burning fossil fuels, by nuclear reactions, or even by using solar energy. (ii) Gas-fueled plants use gas to turn the rotor blades. (iii) Hydro plants use water flow to turn rotor blades. (iv) Different types of renewable energy systems (photovoltaic, wind turbine) etc. The main difference between them is the operating cost and how fast can they respond to quickly changing grid conditions - low operating cost usually means slower response. For example, gas-fueled and hydro power plants are used to control voltage and frequency, i.e. to quickly respond to changes in consumption, while thermal plants usually cover around 80% of the total consumption.
The pumped hydro energy storage (PHES) is the most popular storage type in the power system. The operating principle is simple - when there is excess power pump the water to a reservoir (lake) at higher altitude, and when there is power shortage let the water flow to a reservoir (lake) at lower altitude through big pipes. The water flow will spin the turbine (rotor) of an electric generator which produces electricity on the stator. The round-trip efficiency of these systems is around 70-80%. Other storage examples include compressed air, flywheel, electrochemical batteries, supercapacitors etc. The main difference between them is power and energy density (per unit mass and per unit volume), and of course the price.
Now what happens when there is excess power? Well, the electricity is sold on a market, which means that power producers might offer to pay someone to take the excess power. This phenomenon can be seen around holidays such as Christmas when all industry is shut down for a couple of days and the big power plant operators have realized it is better to pay someone to take the excess power than to shut down and then restart the power plant. I have never heard of someone shunting the power to ground though.
