Let's go back to the beginning first...to understand 'styles' there are multiple links such as http://www.shaunakelly.com/word/styles/stylesms.html that will define styles and how they are used. Typically, styles are not used to bold or underline one particular character within a paragraph...the style applies to the whole paragraph.

It would behove you to take a look at the link provided (and other links available within that link) to learn more about styles and other MSWord attributes.