Here are some views from a buddhist perspective. This will allow us to further broaden our understanding of practice. These three different aspects are very fundamental to the path and not incompatible with other systems. (In fact, we will probably already have an intuitive grasp on them anyway if we have been true to our practice; no matter what system we cleave to).
Concentration is the power of one-pointedness. It can be used to hone awareness into an unwavering line. It is like a focused beam of energy that allows defilements of perception (negative emotions, false personalities, etc. etc.) to be held at bay. This is because it supresses the resulting development and manifestations of the root causes of our perception objects (or subject->object perceptions) through time. However, since concentration is the power of supression it has no effect on the root causes themselves. It is like shutting everything else out except the current focus, thus increasing the amount of energy flowing to the current focus. Concentration is developed through meditation and can be a great help to silencing the mind and also remaining with perception objects (subtle or otherwise). It is an aspect of effort that increases energy flow and can pertain to sense objects, mental objects, and/or the emotions/feelings; it is also the pathway to non-ordinary perceptions in general. So called 'occult' powers that affect the world at large all belong to the power of concentration. Concentration is the essence of unchanging perception. (Self-presence belongs to the power of concentration)
Mindfulness is the power of observation. It is a sweeping awareness which focuses on all objects of perception equally. A good analogy is to compare mindfulness to the sun, which shines on all things equally with no discrimination. Mindfulness does not hold defilements of perception at bay, rather it watches all things, be they defilements, objects of perception, subtle perceptions, or even empty mind. While the power of concentration grows into a progressively more focused energy, the power of mindfulness grows in constancy and space of accomodation. It is an aspect of effort that increases the range of conscious awareness in general. Mindfulness is the essence of equanimous perception. (Self-awareness belongs to the power of mindfulness)
Concentration and mindfulness complement each other in a sort of active-passive effort of practice. Later, when we become more proficient, they can even be performed simultaneously - as we can concentrate on being mindful. In this case, rather than supression of perceptions in general, the power of concentration is used to support the practice of mindfulness with unbending intent by supressing distractions of that effort (i.e. the double negative effect, or negating a negation). If we keep with it, at some points during our practice we will have experiences of insight. Eventually, this will predominate.
Insight is the power of seeing the essence of things. It is like a flash of lightning that illuminates the deeper recesses of awareness. Insight actually has the power to uproot defilements of perception in a total way. It goes to the root cause of perceptions and brings them into conscious awareness. Once the root causes of perceptions are returned to their source, there is no more need for them to manifest as objects. All things arise from and evolve back to the source. Insight is a result of effort that purifies consciousness. Insight is the essence of pure awareness. (Self-knowledge belongs to the power of insight)
