Bootstrap includes six predefined button styles, each serving its own semantic purpose.

Use a class .btn-outline-{color} to quickly create a outline button.

Use .btn-flat-{color} to create a flat button

use btn-gradient-{color} create gradient buttons

Use .btn-relief-{color} to create a relief button.

Use a class .round with outline button class to create round outline button.

You can use .btn-icon. you can create a rounded button by using .rounded-circle with .btn-icon. You can only use .btn-icon when you only want icon in your button

Group a series of buttons together on a single line with the button group. Wrap a series of buttons with .btn in .btn-group.

Create block level buttons—those that span the full width of a parent—by adding .btn-block.

The .btn classes are designed to be used with the <button> element. However, you can also use these classes on <a> or <input> elements.