Concentration puts the mind onto something, but it doesn't always know what it's actually looking at or examining. It's very real that we can be reading a book, the eye sees each word, the mind knows each word, one at a time, we read two pages, and we can't remember what we read. This is a classic case where concentration is present, but there's no awareness, there's no knowing quality in the mind. So awareness actually knows what concentration is focusing on.
Now, the word "concentration" in English is used in different ways, so again we have a problem with language here, but as to the practice, when we talk about concentration, it's putting the mind onto a subject, whether thinking a thought in meditation, or looking at the door before you grab the door, etc. That's the concentration part. The awareness or the mindfulness actually knows what we're concentrating on.
Now to a certain extent it's helpful to know that the word mindfulness, or awareness, in English, comes from a Pali word Sati, and it can also come from a Pali compound word Sati-Sampajanno. And it is valuable to know that there is a difference between these two, between Sati and Sati-Sampajanno. But in English, we always use the same word, we use mindfulness or we use awareness. So this is valuable to know about, because the word Sati can actually be closer to just concentration than concentration is to Sati-Sampajanno.
To explain what that means; imagine that we're sitting, and we feel that we want to practice body awareness, we want to be mindful of feeling our body move. So we think to ourselves, "I'm going to move and raise my right arm and move it in the air." So we're very mindful of raising up our arm, and we feel the muscles in the forearm, in the shoulder, in the neck, and we're very mindful. Then we decide to move it, and we decide to move it to the right, move it away from our body, and fine, we're practicing mindfulness, Sati, mindfulness just in the moment, just knowing what's happening in the moment. But if someone is sitting next to us, we just hit them in the face!
This is not Sati-Sampajanno. Sati-Sampajanno knows more than Sati. Sati-Sampajanno means "Wisdom-in-Action" or "Clear Comprehension." So with Sati-Sampajanno, if I wish to do something like this; then I have to apply both, I want to practice Sati of body movements, and I want to practice Sati-Sampajanno also of my actions. Then it will work more like this: We're sitting here, and we're raising the arm, and we feel every muscle and the movements and so on, and when it gets up in the air, Sati-Sampajanno thinks, "What would be the result if I move it to the right or to the left? Oh, somebody is sitting to my right, I better move it to my left." And so Sati-Sampajanno then basically tells Sati which way to move the arm and continue with its practice.
Sati-Sampajanno is a big broad view which we really want much more than just concentration or just Sati type mindfulness. We want Sati-Sampajanno, it's got the wisdom, the understanding to know actions and results of actions. Sati by itself doesn't know that. A good thief who's been robbing banks in the middle of Munich for the last 15 years, has wonderful concentration, has wonderful Sati type mindfulness, but they do not have Sati-Sampajanno, they don't understand that the results of what they're doing is harmful, and will eventually harm themselves as well.