html classes question

Recent versions of zen cart use a number of new classes to style output but some are strange to me and I don't know what they mean or when to apply. For example, col-xs-12, col-sm-3 and similar, btn, fa, fa-angle-right, etc. Can someone explain what these mean or point me to some documentation?

Thanks.

Those fa-type classes identify Font Awesome glyphs (https://fontawesome.com/start) and the col-* classes come from Bootstrap (https://www.w3schools.com/bootstrap4...ll_classes.asp).

Thanks lat9! Always learning something.