What is the difference between ID and CLASS?

ID identifies and sets style to one and only one occurrence of an element while class can be attached to any number of elements. By singling out one occurrence of an element the unique value can be declared to said element.

#eva1 {background: red; color: white}
.eva2 {background: red; color: white}

<P ID=eva1>Paragraph 1 - ONLY THIS occurrence of the element P (or single occurrence of some other element) can be identified as eva1</P>
<P ID=eva1>Paragraph 2 - This occurrence of the element P CANNOT be identified as eva1</P>

<P class=eva2>Paragraph 1 - This occurrence of the element P can be classified as eva2</P>
<P class=eva2>Paragraph 2 - And so can this, as well as occurrences of any other element, </P>

How to make text-links without underline?
a:link, a:visited {text-decoration: none}
<a style="text-decoration: none" HREF="...">
...will show the links without underlining. However, suppressing the underlining of links isn't a very smart idea as most people are used to having them underlined. Also, such links are not spotted unless someone coincidentally runs a mouse over them. If, for whatever reason, links without underline are required background and foreground colors can be instead declared to them so that they can be distinguished from other text, e.g.;
a:link, a:visited {text-decoration: none; background: red; color: blue}
<a style="text-decoration: none; background: red; color: blue" HREF="...">

Both background and foreground colors should be specified as the property that is not specified can be overridden by user's own settings.