SASS is a powerful tool in the modern developer’s toolkit. Syntactically Awesome Style Sheets, have been around, in some form or another, for over a decade. SASS can create easy-to-maintain, updatable, streamlined, and DRY CSS.
BEM stands for Block, Element, Modifier and is a popular semantic method to create naming classes for your HTML DOM objects. It takes some of the work out of selecting appropriate classnames (one of the most difficult tasks a developer faces).
Shameless self-promotion: I will be on the PHP Roundtable podcast February 22nd! The show will cover GitHub, contributing to open source, and other related topics. You can watch live or view the post-show recording on YouTube.