Bootstrap includes six predefined button styles, each serving its own semantic purpose.
Border
Use a class .btn-outline-{color} to quickly create a outline button.
Flat
Use .btn-flat-{color} to create a flat button
Gradient
use btn-gradient-{color} create gradient buttons
Relief
Use .btn-relief-{color} to create a relief button.
Round buttons
Use a class .round with outline button class to create round outline button.
Icon
Icon Only
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
Basic Button group
Group a series of buttons together on a single line with the button group. Wrap a series of buttons with
.btn in .btn-group.
Checkbox & radio group
Sizes
Add .btn-lg or .btn-sm for Fancy larger or smaller buttons size.
Add .btn-lg or .btn-sm with .btn-outline-* for outline btn in
different sizes
Block level buttons
Create block level buttons—those that span the full width of a parent—by adding .btn-block.
Button tags
The .btn classes are designed to be used with the <button> element. However,
you can also use these classes on <a> or <input> elements.