The "Internal Style Sheet" is defined and implemented only within an HTML document.

Creating CSS style sheets:
(i) Open an empty notepad.
(ii) Type the style properties and their values
(iii) Save the file with extension .css.

"Inline style is used to define style for a particular tag anywhere in an HTML document. You can define styles for any tag within an HTML document. But it is applicable only on that line where it is defined.

A style tag is used to change the default characteristics of a particular tag in the entire web document wherever that tag is used. The < style > tag controls the presentation styles of a particular HTML document. If you want to use a particular tag with the same style applied in one HTML document to another is not possible.
Thus, the < style > tags are called as "Page-Level Styles" or "Internal Style Sheets".

Linking CSS with HTML:
The < link > tag is used to add CSSfile with HTML in head section. While using < link > tag, the following attributes are also included along with standard values.
rel = "stylesheet"
type = "text/css"
The href attribute is used to link the .css file.
General format of < Link > tag
< Link rel = "stylesheet" type = "text/css" href = CSS_File_Name_with_Extension >
HTML Code for Formatting HTML element through CSS:
Mystyle.css
H1
{
font-family: "Comic Sans MS";
Font-weight: Bold;
border:2px solid blue;·
}
P {
font-style : Italic;
color :MediumSeaGreen;
}
--- CSS_Test.htm --- 
< html >
< head >
< title > Demonstration of using CSS < /title >
< link rel="stylesheet" type="text/css"
href= "mystyle.css" >
< /head >
< body >
< H1 > Welcome to CSS < /H1 >
< p >
CSS was invented by HakonWium Lie on October 10, 1994 and maintained through a group of people within the W3C called the C5S Working Group. The CSSWorking Group creates documents called specifications. When a specification has been discussed and officially ratified by W3C members, it becomes a recommendation.
< /p >
< body >
< html >

H1
{
Font-family
}
Font-family : "Arial";
        Font-style : Italic;
        Font-size : 24px;
        }

{
color: red; Text-align : center;
}

(i) Maintainability : CSS are also defined and stored as separate files. So, the style and appearance of a web page can be dynamically changed and maintained with less effort. 
(ii) Reusability : The styles defined in CSS can be reused in multiple HTML pages.
(iii) Easy to understand : The tags in web pages are well organized with style specifications and therefor it is easy to understand.

< html >
< head > < title > CSS DOCUMENT < /title >
< style >
P
{
font-family : "Cooper Black"; Font-style: Bold: Font-weight: Italics; Color: Blue;
}
< /style >
< body >
< p >
The State Institute of Education (SIE) was established in 1965 to provide for systematic study of problems relating to School Education under the administration of Directorate of School Education.
< /p >
< /body >
< /html >