Bold for emphasis with ** or ⌘+B or the B button.

Italic for subtle nuances with * or ⌘+I or the I button.

Strikethrough to show revisions with ~~ or the ~~S~~ button.

Code for code snippets with : or ⌘+⇧+C or the C button.

Underline for emphasis with ⌘+U or the U button.

Superscript for footnotes with ⌘+. or the . button.

Subscript for chemical formulas with ⌘+, or the , button.