In html or css style sheet, What is the difference between id and class ?

Answer 1

id - unique name of element
class - can be used by a group of element. Reusable.

ID symbol - #
Class symbol - .

Example:
#demo - selects only one element with a "demo" id.
.demo - selects elements with the same class "demo".

Answer 2

The id selector uses the id attribute of an HTML element to select a specific element. The id of an element should be unique within a page, so the id selector is used to select one unique element!
To select an element with a specific id, write a hash (#) character, followed by the id of the element.
The style rule below will be applied to the HTML element with id="para1":

#para1 {
text-align: center;
color: red;
}

The class selector selects elements with a specific class attribute. To select elements with a specific class, write a period (.) character, followed by the name of the class.
In the example below, all HTML elements with class="center" will be red and center-aligned:

.center {
text-align: center;
color: red;
}

https://www.w3schools.com/css/css_syntax.asp

Answer 3

thank you both of you