https://github.com/tpope/vim-surround/blob/master/README.markdown

It's easiest to explain with examples. Press cs"' inside

"Hello world!"

to change it to

'Hello world!'

Now press cs'<q> to change it to

<q>Hello world!</q>

To go full circle, press cst" to get

"Hello world!"

To remove the delimiters entirely, press ds".

Hello world!

Now with the cursor on "Hello", press ysiw] (iw is a text object).

[Hello] world!

Let's make that braces and add some space (use } instead of { for no space): cs]{

{ Hello } world!

Now wrap the entire line in parentheses with yssb or yss).

({ Hello } world!)

Revert to the original text: ds{ds)

Hello world!

Emphasize hello: ysiw<em>

<em>Hello</em> world!

Finally, let's try out visual mode. Press a capital V (for linewise visual mode) followed by S<p class="important">.

<p class="important">
  <em>Hello</em> world!
</p>

This plugin is very powerful for HTML and XML editing, a niche which currently seems underfilled in Vim land. (As opposed to HTML/XML inserting, for which many plugins are available). Adding, changing, and removing pairs of tags simultaneously is a breeze.

The . command will work with ds, cs, and yss if you install repeat.vim.