Several questions about Class in Unity

I am watching the "Space Shooter tutorial" to learn using Unity Engine.In the part 14"Counting Points"the tutorial teacher has a problem that he is addressing the class directly but not a specific instance of the game. Therefore,I started confusing of "What is a class?" Although I found many video said that"classes are functions and variables you can easily call" or "A script is a task,when you are scripting ,you want to make sure each script you have a specific task.",I still not understand what is classes. So,here are the questions I want to know :

1.What is a class and what is the class usage? 2.Someone said class is a script,is it true? 3.What is a instance means?The tutorial teacher have used the word many times.......

Maybe there are many gammer problems in this question(because I am a "HongKongese".....) I will be so grateful if my question be soloved. BTW Happy New Year for all Unity Users^_^