A dialog is a small window that overlays (pops up over) the content of a webpage. Dialogs communicate information to a user and can prompt them with questions or actions that do not require a response.
Use a modal component when you have a limited amount of space in a page, but still want to request optional information from a user. A modal is meant to contain another component and does not have its own properties. In Sitecore, the modal can be used on any link or button. Components that can go in the dialog include forms and content.