Styles in the Rich Text editor are limited - you can either define a style on a generic tag (H1, H2, P etc.) or as a named class which will wrap whatever you have highlighted in a span with the class name applied.

Examples:

If I have a class named .quote I can do this:

/**umb_name:Quote*/
.quote {
    font-weight: bold;
}

.quote:after {     
    content: '\201D';
}

.quote:before {
    content: '\201C';
}

Then when I apply the style to a selected word or words (highlighted below) in a paragraph like so:

The quick fox jumped over the lazy dog

the html becomes

<p>The quick fox <span class="quote">jumped over the</span> lazy dog</p>

and should in theory be rendered something like this:

The quick fox "jumped over the" lazy dog

You can also create styles that change the element of a block of text by specifying the element as the selector like so:

/**umb_name:Heading 1*/
H2 {
}

/**umb_name:Heading 2*/
H3 {
}

/**umb_name:Body Text*/
p {
}

If you then click on a paragraph and choose the "Heading 1" style from the drop down the paragraph will be converted to a H2 with the default styling.

It's worth noting that not all tags can be converted or have classes applied - img tags for example are ignored.

Hopefully this helps clarify things...