If you haven't got a correct definition of Potential Difference, Current and Resistance in your head then I suggest you stop reading now and revisit electricity at GSCE level! Don't take that the wrong way, but in Electricity at A Level you are going to come across a whole lot new quantities, and these three are crucial to understand before you build on the other ones. I think a great deal of students find this topic hard because they have never got a useful model of electricity firmly in their mind. They've just muddled through GCSE with good maths skills to solve electricity problems! You need to build new understanding on solid, technically correct conceptions, otherwise misconceptions will arise!
The three quantities; Potential Difference (often called Voltage) , Current and Resistance are of course linked by Ohm's Law. Here are their definitions:
Potential Difference - The difference in energy per unit charge between two points in a ciruit.
Current - The rate of flow of charge with respect to time.
Resistance - A measure of a component's opposition to flow of charge.
Did any of those definitions surprise you, if they did, take your time, go back and look at the GCSE section on electricity, look again at the models of electricity that you will have studied as far back as when you were twelve or thirteen years old!
The same can be said about Kirchoff's Laws, which tell us how to combine these quantities in series and parallel electric circuits. You need to be able to calculate quantities in tricky looking circuits accurately if you are going to be a success in this section of the A Level curriculum.