A common mistake when talking about SEO is to use the term "optimize your title tags". This is an incorrect use of the word tags.

HTML is made up of elements. Elements have start tags and ending tags.

This is an HTML title tag: <title>

This is an HTML title element: <title>This is a title</title>

The following image shows the start and end tags on a title element:

The W3C talks about it:

Elements are not tags. Some people refer to elements as tags (e.g., “the P tag”). Remember that the element is one thing, and the tag (be it start or end tag) is another.

When talking about SEO, use the phrase, "optimize your title elements".

Don’t Optimize Your Alt Tags Either

Another common mistake is to call attributes tags. For example, "optimize your alt tags".

Google shows over 800,000 Web pages that contain the text "alt tags" (probably incorrectly).

You should not add alt tags to your images — you should add alt attributes to your images.

Wikipedia has a good illustration that summarizes the sections of an HTML element:

I also recommend reading the Wikipedia page on HTML elements.