If you are webmaster and not sure which HTML elements are important in website optimization then you must read this post.
Bold Tag < bold >< /bold >: This tag is important only if you want to make a special word more stylish it will nothing to do with SEO. If you bold your keywords then search engine will only consider it as stylish font and will not give any importance.
Strong Tag < strong >< /strong >: This tag represents important of particular word. If you want to optimize your website for specific keyword then use < strong > tag instead of < b >.
Italic Tag < i >< /i >: This tag represent a word that is represent alternative word or mood. Use this tag to identify a name, technical term, designation, natural phrase from other language, etc…
Emphasis < em >< /em >: This tag is different then Italic Tag because the Emphasis represent the word that is intend to stand out of the paragraph. If you want to give an important to word then strong tag is more appropriate.