Some of the simplest ways to make text look nicer, or have a better impact, is using bold, italic, or underline. I wouldn't reccomend having your whole page full of bold text, but one thing you can use bold, italic, and underline for is making your text standout better. Or, you could format parts of your text that you want visitors to notice. For example, let's say you have a gaming website. Compare these two peices of news:

Microsoft announced today that XBox...

Microsoft announced today that XBox...

Notice how XBox stands out more because it is italic? It makes sure visitors notice it. Important things in a story could be italisized, or if you want to make something really stand out, you could use bold or underline. Like, perhaps you have categories with things in them. You'd want the category names to stand out, so that people would know they were categories. You could make the header of each category bold. Now, enough rambling on about what you'd use these for... here's how to use them.

It's really easy. For bold you use <b> to open the tag, and <b> to close it. As you probably know, the way HTML works is that with most tags, you have an opening tag and a closing tag. Anything in between the opening and closing tags is formatted, depending on what tag it's in between. For italic, your tags are <i> and <i> to close it. Can you guess what underline is? If you guessed <u> to open the tag and <u> to close it, you're right!