Self-defense or self defence depending on where you come from is spelled a little differently
, however both words mean the same thing. Self defence is a counter measure that involves protecting yourself, your property or a loved one from being harmed by another person or persons.
To be protected by the law when using self defence you must prove that the other person was going to cause you physical harm, this can be done in a variety of ways including describing their demeanour, using CCTV or from other witnesses.
Many fighting styles teach moves that are suitable for self defence and most teach to block and then counter without being hit but sometimes this is impossible so it's best to protect yourself as much as possible and then hit. I also believe in making the first hit, yes this is self defence so you would assume that you need the other person to hit you first, however if you believe that the person is going to hit you and cause damage then why wait? Make the first strike and try to end the fight as soon as possible.
Some self defence arts also teach their students to use verbal methods to calm the situation down, however most of the time if a person is wound up or angry enough using verbal techniques isn't going to work, in instances like this it is very useful to know a self defence system such as Krav Maga.
If you would like to learn more about self defence then I highly recommend checking out some videos or self defence books which can explain how to defend yourself in a variety of different situations.