Technically, it's the highest bid
that has been accepted by eBay before the auction closes. The last bid
may also be the winning bid, but if you get your max in there and it holds up it doesn't matter what the last bidder does. And if your bid doesn't hold up, well, somebody else was willing to pay more than you were. Another widgit just like the one you were bidding on will turn up on eBay eventually.
The point is that by cutting your lead time too close you reduce the chances of your snipe bid getting through eBay's servers in time geometrically. Yes, eBay uses more than one server to process bids and yes, eBay has a lot of servers...but do you know how
many bids eBay has to handle per second? If you've ever been looking around on eBay and it takes like forever to get from point A to point B, you're seeing an example of eBay's servers getting to the breaking point.
A proxy bid is when your bid is higher than the amount needed to be the high bidder of record. Say the current bid is at $50 and the bidder's maximum bid was actually $60, the $10 difference is held in reserve as a "proxy" by eBay so that if and as other bids come in, his bid is automatically adjusted upwards until his max is met. Now let's say you join the fray with a $100 bid. You'll show up as the high bidder with $61; the other $39 will be held in proxy reserve just like the last guy's was.