A few different ways to describe CST - from both patients and practitioners - can be found on the Craniosacral Therapy Association website. As you can see, there are many different ways to describe this multifaceted approach to health.
Originally, CST was a branch of Cranial Osteopathy, and so has this tradition running strongly through its veins. Although you may see in some descriptions that CST is about working with cranial rhythms, my personal view is that it is not limited to rhythms or to fluids. It is a way of thinking about health that encompasses the whole body - and the whole body-mind, the whole person. So, whilst rhythm, tide and fluid forces are a common thread in the practitioner's experience, they come and go, rise and fall, in a sea of health.
CST is not manipulation or massage or use of pressure points, but it still a way of working with the physical body. Cranial practitioners feel movements in the body and use this for diagnosis, and also "latch onto" those movements to help the body heal itself. There is more trust in how the wisdom of the body expresses than in any specific technique or analysis of "what might be wrong". Most of CST focusses on literally assisting the body's own inherent self-healing potential. This makes it very efficient - because instead of one person in the room trying to make something happen (by "do-ing" something to another person), we have two living beings cooperating with each other.
As such, CST could be described as a "conversation" between the therapist's hands and the body. Bodies are intelligent organisms that have evolved with very powerful self-repair abilities. We feel for the signs of health - be that in rhythms or small movements, and act as facilitators so that the body can find its own solution. And with deeper stillness, there is a sense of connection to reservoirs of health that are waiting to bubble up through layers of accumulated time. It is, in the words of Carlos Castenada, a "Path with a Heart". This waiting for the body takes some time, but the waiting is worthwhile. It is not a struggle that includes the use of potentially dangerous force - CST is one of the safest therapies. And when "something happens", it happens with ease and often in ways not expected.
As I have said, CST may be a little different from most approaches, but the clinical experience is that it works very well, and often remarkably quickly in many cases. Like all types of medical intervention, there is not 100% success - that would be impossible. But I have seen excellent "results" with many different "conditions". See here for more information on the types of health problems I believe it has helped. Another description of CST can be found on Linkedin.