difference between br (line break) tag and paragraph tag

<p>This is first paragraph.</p> <p>This is second paragraph.</p> <!--output will be with line with space--> This is first paragraph. This is second paragraph. <!--in case of (<br/> also called empty tag because no closed tag)line break without space--> This is first paragraph.<br/> This is second paragraph. <!--output--> This is first paragraph. This is second paragraph.