btn: This class is used to create a basic button.
btn-primary, btn-secondary, btn-success, btn-danger, btn-warning, btn-info, and btn-light: These classes are used to add different colors to the button.
btn-lg, btn-sm, and btn-block: These classes are used to change the size and display of the button. btn-lg creates a larger button, btn-sm creates a smaller button, and btn-block creates a block-level button that spans the full width of its parent.
btn-outline-primary, btn-outline-secondary, btn-outline-success, btn-outline-danger, btn-outline-warning, btn-outline-info, and btn-outline-light: These classes are used to create an outline button with a colored border.
btn-link: This class is used to create a button that looks like a link.