<U> </U>

The U tag is that the text inside it should be underlined. Most browsers use underlined text to indicate hyperlinks, so you may want to avoid using this. It is possible that it will confuse some people when the "hyperlinks", which are actually just underlined text, don't work. Underlining is mainly used as an alternative to italicized text (for example, on typewriters). Since HTML has a separate tag for italics, you may want to use that if this is why you are using the U tag. Most graphical browsers don't support underlined text, because it makes it harder to distinguish normal text from hyperlinks, so this tag will not always work.

<u>This is underlined text.</u>

This is underlined text.