Parallel circuits are circuits in which there are multiple paths running alongside one-another. In many cases this will be due to either multiple power sources flowing to a single output, or one power source running to multiple outputs. By splitting the circuit along several lines in this way, parallel circuits are able to achieve things series circuits cannot, but they also come with several disadvantages.
No matter whether you have several power sources or just one, the voltage across a parallel circuit will remain the same. This is because the potential difference, which is the source of voltage, is split across the circuit as a whole; so increasing the power will not alter the voltage.
In a parallel circuit, the energy from the power source is split across the circuit. This results in different levels of current flowing, depending on where in the circuit you place an ammeter to check it. This prevents parallel circuits from being used anywhere a constant current is required throughout.
When branches are added to a parallel circuit, the voltage is equalised across the whole circuit, meaning the current flow must change to compensate. This has a knock-on effect on the resistance in the circuit as a whole and results in a lower resistance in the circuit when more resistors are added in new branches. The only way to increase resistance is to add resistors in series with one another and on existing branches.