A normal bid is one where you wish to raise the current bid to your maximum bid immediately. So, if you placed a normal bid of $15, the bid would immediately become $15, regardless of what the next lowest bid is. The software is configured to only do proxy bids, when your bid is more than $2 higher than the next lowest bid.

A proxy bid is one where your bid is significantly higher than the current bid, but you do not wish to pay more than one advance over the underbid. In this example if you placed a $15 proxy bid and the current bid is $5, it will advance the bid by only $1, taking it to $6, until someone bids more. The software acts as your agent, bidding only as necessary.