Tags

Tags are the link between your post and other elements of Wordpress.  You use a keyword as the tag and then later at the post itself you connect the tag to the post.  You can also add a tag directly from the post page, that will add the tag to the Post Tags list.

Posts tags page

As you can see above you are not limited in your choice of keyword for the tag.  Tags are different that categories – one way they are different is tags do not relate to each other as categories can since categories can be hierarchical.  Tags can take you dirctly to a post where you might, as a reader, use a hierarchy to take you to a group of posts.

When using the WordPress Default Theme, you can see the Tags that is assigned to a post when you display the posts.
The Edit Tag window is shown below and as you can see the Tag editing is free form and simple to use.

As mentioned on the posts page, Tags are also a method used to assign the post to specific pages within a web site.