Ordered and unordered lists are the two main types of lists in HTML. Ordered lists use the <OL> tag and display items in numerical or alphabetical order, while unordered lists use the <UL> tag and display items with bullet points. The <LI> tag is used within both <OL> and <UL> tags to specify each list item. The TYPE and START attributes can be used with the <OL> tag to customize the numbering or lettering of ordered lists.