Mains electricity

• Fuses: Stop the flow of current by melting if the current is too high. So protecting sensitive components and people because if the components function at too higher temperature it can cause a fire. 
• Circuit breakers: again break the circuit if current is too high. 
• Insulation and double insulation prevent people from touching exposed wires and getting shocks.
• Earthing provides a low resistance path to the earth so if some one does come into contact with a current instead of flowing through them to the earth giving them a shock it flows through the earthing wire.  
• Resistance causes transfer of electrical energy to heat energy. Some components are designed to have a high resistance to make sure this happens, for example electrical heaters that have lots of resistors to ensure a high resistance so a lot of heat is produced.
• power (w) = current (A) x voltage (V) (P=IV)

  when looking at a circuit a component will be given a power and a voltage appropriate to run at then the current can be calculated so the rating of the fuse can be selected for slightly higher than that. 
• Energy (J) = potential difference (V) x current (A) x Time (s) 

  E=VIt
• AC is constantly changing magnitude and direction. AC is how mains electricity is produced from turbines.
• DC is constant. And is produced from a battery and used in some sensitive components like in computing.