Can you explain the differences between Bootstrap's pre-defined classes and Less variables?

Bootstrap uses both pre-defined classes and Less variables to control the styling of a website. The main difference between the two is the way they are used and the level of control they provide.

Pre-defined classes are ready-made CSS classes that you can apply to elements on your website by adding them to the HTML markup. They are designed to be simple and easy to use, and they allow you to quickly change the layout, alignment, and other visual aspects of your website. For example, the class .text-danger will change the text color to red.

Less variables, on the other hand, are used to store values that can be reused throughout the stylesheet. They allow you to change the default values of Bootstrap's CSS properties, such as colors, fonts, and spacing. This means that you can customize the look and feel of your website without having to override the default styles. For example, you can change the primary color of your website by changing the value of the @brand-primary variable.

In general, pre-defined classes are simpler to use, but Less variables give you more control over the styling of your website. You can also use both pre-defined classes and Less variables together in your project.

It's also worth noting that Bootstrap uses SASS instead of LESS, but the concept is the same.