HTML markup largely ignores line breaks within marked up files. Instead, if you want to insert a line breaks you need to insert a <br> tag.
When a line break is inserted the cursor moves down a single line, which is different from the paragraph which ends the paragraph and starts a new one. When you hold Shift and press Enter a line break tag is inserted ( <br /> ) and the text entered after the line break will appear on the next line down.
For example the following markup: