In HTML, there are three main types of lists that can be used to organize and structure content: 1. Ordered List (`<ol>`): An ordered list is used when the order of the items is important. Each item is represented by an `<li>` (list item) element, and by default, the items are numbered sequentially. The numbering style can be customized using CSS. Here's an example of an ordered list: 

<ol>
  <li>First item</li>
  <li>Second item</li>
  <li>Third item</li>
</ol>
2. Unordered List (`<ul>`): An unordered list is used when the order of the items is not important. Like the ordered list, each item is represented by an `<li>` element, but instead of numbers, bullet points or other markers are used to denote the items. Here's an example of an unordered list: 

<ul>
  <li>Red</li>
  <li>Green</li>
  <li>Blue</li>
</ul>
3. Definition List (`<dl>`): A definition list is used to present a list of terms and their corresponding definitions. It consists of `<dt>` (definition term) elements for the terms and `
` (definition description) elements for the definitions. Here's an example of a definition list: 

<dl>
  <dt>HTML</dt>
  <dd>HyperText Markup Language</dd>
  <dt>CSS</dt>
  <dd>Cascading Style Sheets</dd>
</dl>
These list types can be nested within each other to create more complex structures. For example, you can have an ordered list within an unordered list or vice versa. Additionally, CSS can be used to modify the appearance of the lists, such as changing the bullet points or numbering style, adjusting the spacing, or applying custom styles.