If we were to take a look at the mixins available for use in Bootstrap, we can see they are broken down into a number of groups. These groups are:
Core variables and mixins
Resets and dependencies
Core CSS
Component files
Component files that use JavaScript
Utility classes
Let's take a brief look at each one in more detail, starting with the core variables and mixins.
Core variables and mixins is probably one of the smallest groups of all of the mixins but is the most crucial—it contains variables.less, which lists all of the predefined variables available for use within Bootstrap. The second file contained within this group is mixins.less. While it is simpler in design, it plays an equally important role: it contains the @import statements that are used to build the bootstrap-theme.css file.