Reversals: A reversal creates a brief window where you cannot block some attack types, but will automatically stop others. Some reversals like Asuka's take the form of inescapable throws when successful. Others, like Law's, cause a stun and allow the reverser to get a free hit. The most common reversal is a short-lived punch reversal for mids and highs. Steve, Paul, Hwoarang and others have this.
Reversals can commonly be punished on reaction if whiffed. Most reversals cannot reverse knees, elbows, shoulders or headbutts.
Sabakis
A sabaki is an attack that's simultaneously a reversal. Instead of being counterhit by a particular kind of attack, you'll reverse it. Worth noting is that the reversal frames usually do not last throughout the entire sabaki attack. For example, a 23 frame punch sabaki may have only have reversal properties for its first 6 frames. Any attack, including punches, will counterhit it as normal outside these frames.