These tags are the tags that keep your page together. They define the paragraphs, spaces, and quotes within your page.
<p> - the paragraph tag
This is the paragraph tag. The ending tag </p> is optional in HTML and required in XHTML. Use the paragraph tag anywhere you want a blank line after some text. One thing to remember is that you cannot use multiple paragraph tags in a row. The browsers will simply ignore all but the first tag.
<br> - the line break tag
This is the line break tag. There is no ending tag. Use the <br> tag anywhere you want your text to start on the next line (without a space). Most browsers will assign space for every <br> tag that you use, so if you want a large gap of white space, you can use several <br> tags in a row. (Note: this isn't considered good style, but you can do it.)
<blockquote> - the quotations tag
This is the quotations tag. The ending tag </blockquote> is required. Use this tag to indent quotations a few spaces (how many is up to the browser). If you nest this tag (use several of them in a row), the text will be further indented. Some browsers indent on both sides of the screen, and some only indent on the left.
Learn more about these and other tags in the HTML Tag Library.