Prison Architect is forever trapped between two political poles: the side that says that prisoners should be locked up, punished, and left to rot; and the side that thinks they should be reformed, educated, and made better able to return to society and not re-offend. Introversion want both methods to have value within their management game, and alpha 18 takes the first steps towards enabling the liberal half by adding therapists.
Also tazers. New update video below.
Therapy is the first of a spread of planned methods for reforming your prisoners. It's designed to be a hard thing; it takes five, two-hour sessions with a therapist for a prisoner to show any progress, and even if they attend all five without being shivved in the showers, there's no guarantee the process will work. If it does though, it'll reduce that prisoner's "boiling point", the stage at which they turn Hulk and start acting violently.
As part of the reformation process, you might want to educate your prisoners in various trade skills to help them after they leave your prison walls. Previously you could send any old inmate into your workshop and kitchen and instantly have them stamping out number plates for you to sell, but now you need to send them to a class program first which will teach them how to use the machinery and stop them from slicing their own hands off in the kitchen. Beyond that, you can send your prisoners to general education programs, which involves building them a classroom and bringing in a teacher each day to lead the class.
If all this education lark sounds technical, no worries. The update also expands alpha 17's armoury, introducing tazers alongside the previous deadly weaponry. You can distribute those tazers among your guards and dog handlers and use them as a non-deadly way of incapacitating unruly prisoners.
You can read a full list of changes over at the Introversion blog.