What is id attribute and class attribute and what is the difference between it?

Answer 1

Long story short:

ID is (or should be) unique and references one single HTML element.

Class describes a type of an element or a structure which that element belongs to.

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ID gives an (HTML) element (tag) a specific "reference/address"
One rule is that an ID should be unique (eg.: if you got a DIV element with ID "myDiv" there should be no other element (of any kind) with the same ID.

Think of it as a person. Each person is unique. There might be a doubleganger (similar looking), but still it's not the same person.

Hence, an element with an ID can be easily referenced with (Javascript for example) without further information.

A Class is mainly used for styling with CSS. It gives the elements a structure. Class attributes follow other rules.
A class attribute can contain multiple values separated by spaces (that means that the element belongs to different classes). Besides that a class doesn't need to be unique. There may be several elements with the same class.

Answer 2

//Id and class are selector of css which are used to select html element.

The difference between an Id and a class is that an ID can be used to identify one element, whereas a class can be used to identify more than one.