To save you from having to type paragraph tags in your posts and pages, WordPress automatically converts line breaks into paragraphs and new lines into line breaks. Explanation:
In the HTML editor
Leaving an empty line between 2 lines will automatically convert them into separate paragraphs, while continuing on the next line will convert it into a line break. Typing the following..
Here is a paragraph of text. Here is the 2nd paragraph, separated by an empty line. This is a new paragraph with a line break
..will result in the following HTML output:
<p>Here is a paragraph of text.</p> <p>Here is the 2nd paragraph, separated by an empty line.</p> <p>This is a new paragraph<br/> with a line break</p>
In the Visual editor
Hitting the Enter (Return) key will end the paragraph you are writing and start a new one. If you want to start a new line without creating a new paragraph, press the Shift key while hitting Enter.
Keep in mind that if you write paragraph tags (<p>) in the HTML editor and switch to the Visual editor and back, WordPress will strip out the tags. If you only want to write HTML for a page or post, consider using the Disable Visual Editor WYSIWYG plugin.