Hi Mindscape,
Ok, my understanding... without thinking too much about it...
Focus is placing awareness on a single point.
Concentration is maintaining focus over a period of time.
For example, we may place our focus on our breath, and with true concentration, our focus will remain on our breath... however, when the mind gets in there and starts playing with us, it starts throwing thoughts at us and knocking us off our focus, thus we lose concentration. In some subtle sense this may be that the mind has started to introduce thoughts about how deep or shallow the breath is, or how regular the breath is, and then our focus turns to these thoughts and we end up starting to try and force our breath to match the thoughts.... which isn't letting the breath happen naturally, and isn't simply being focused on (aware of) the breath.
That's just an example, you can apply the same to anything.
Hugs
Giles