These are headings. The smaller the number, the larger the font size the browser will render the heading in.

This is the paragraph tag. Browsers automatically add some space (margin) before and after each <p> element. The margins can be modified with CSS (with the margin properties).

The span tag by itself has no visual effect but it is extremely useful when you need to style just a portion of text.

You can use it like this:

<h3>Example</h3>
<p>This is a paragraph with one <span class="blue">blue</span>word</span>