How to use Tags

I am trying to work out the correct usage of tags. I have read some articles, but am yet to find something that explains them clearly.

From what I can tell, there are various uses for tags. Sometimes they are used to link to different areas of the website or to send visitors to a new website.

Is the basic use of tags just for keywords, with the options of having those tags link to other places?

If someone could sum it up for me or link me to a decent guide, that would be fantastic.

Thanks