Pressing the <ENTER> key creates a <P> tag, or a new Paragraph, in Joomla! This will display an extra line space between the previous paragraph and the new paragraph (see example at lower left). Based on your website's configuration, you will probably see a small "P" to the upper left of each paragraph. This will not appear on the front-end of the website.
Pressing <SHIFT>+<ENTER> keys together creates a <br/> tag, or a Line Break, where there is no spacing in between the lines (see example at lower right).
If you apply a style to a paragraph, it will not carry over to the next paragraph.
If you apply a style to a line of text separated from other lines by line breaks, the style will affect all the text separated by line breaks.