Checkpoint types

Hi,

what is the difference between
thread checkpoint,
database checkpoint and
datafile checkpoint?

I read those things in Oracle doc,but i couldn't understand,
Can anyone explain these things with a simple example...

Thanks...

Hi,


In oracle 10g documentation,they've given that,
"DBWn periodically writes buffers to advance the checkpoint, which is the position in the redo thread (log) from which instance recovery begins. This log position is determined by the oldest dirty buffer in the buffer cache."

In this I didn't understand the this line,"This log position is determined by the oldest dirty buffer in the buffer cache.",.What does it mean,can anyone explain how oldest dirty buffer determines log position?

Thanks...