The UL element is for grouping a collection of items that do not have a numerical ordering, and their order in the list is meaningless. Typically, unordered-list items are displayed with a bullet, which can be of several forms, like a dot, a circle, or a square. The bullet style is not defined in the HTML description of the page, but in its associated CSS, using the list-style property
The ul and ol elements may be nested as deeply as desired. Moreover, the nested lists may alternate between ul and ol without restriction.
The ul and ol elements both represent a list of items. They differ in that, with the ol element, the order is meaningful. As a rule of thumb to determine which one to use, try changing the order of the list items; if the meaning is changed, the ol element should be used, otherwise you can use ul.