A HTML page can consist of many different paragraphs.
Paragraphs are separated from each other by some vertical space. Since HTML is white space in-sensitive, <p> and </p> tags help in separating text into groups of related sentences so that they are easy to comprehend.
This is the third paragraph in this sample HTML document. One can clearly see two paragraphs displayed before this paragraph.
This is left aligned.
This is center aligned.
This is right aligned.
This is justified. That is both left and right borders of different lines in this paragraph are aligned. To accomplish this alignment spaces between words are increased in lines with fewer / thinner characters. The space increase after sentence is finished, i.e. after full stops is generally more then space increase between words.