Here's basically how it breaks out (here at booth anyway)
Just about every firm (a few exceptions sometimes exist) will interview X candidates as interview invites and Y (where Y is generally quite a bit smaller than X, but sometimes as much as X) candidates as open candidates. Round here, we call the X people "closed listed" - meaning they made the closed list. There's no set ratio of X:Y -- sometimes you'll see it approach 1:1 and sometimes its 10:1.
From there, you get a fixed amount of bid points you can allocate as you see fit. Here, its 1000 (the number doesn't matter really, it could be 10,000 and all the prices would just increase appropriately). That 1000 points you can allocate any which way you want to firms that haven't closed listed you and you just have to think ahead of how you want to handle that.
So here's a hypothetical example: You apply to 5 firms. Lets say that in week 1, 3 of those firms release their decisions. In week 2, the other 2 will.
Week 1:
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The three firms releasing their decision might be, lets say, Apple, Microsoft and Sun. You get Apple, but Microsoft and Sun don't closed list you. You go online and look them up to see if you can bid on them. Lets say you look em up and you see the following for Microsoft:
# Length Date(s) Open/Invitational Slots
1 30 Tue 18-Nov-2008 Open 4
2 30 Tue 18-Nov-2008 Invitational 11
(BTW, I'm pulling a random job here, its not one of the three actually mentioned). That means they invited 11 (but not you) and there are 4 slots avaliable for you to bid on. Here at Booth you can see the bid history from last year (if the same job existed) so here you see:
Slots: 4; Bids: 16; Price: 500; Min/Average/Max bid: 8/326/775
That means that, last year, 16 people bid on this job - 4 people got it, and the average bid was 326 but you needed to bid 500 to get it. Thats actually quite high.
Lets say you then look at the next job you didn't get -- Sun and you see something similar but now the bid history says
Slots: 3; Bids: 10; Price: 60; Min/Average/Max bid: 1/48/135
That means that last year, Sun was "cheap". You could get a slot for just 60 points. (Again the numbers/data here is made up, I'm not even sure Sun is in business anymore, is it?).
Now you have to think through things:
1. You know that next week you find out from 2 more firms. Are those more important? If you don't get them, how many bid points will you need?
2. If you bid on these and get them both can you prepare?
3. You notice that Sun's interview is on the Friday after Apples. If you get a second round for Apple it might conflict with Suns -- and here's the thing, if you bid, win and don't cancel soon enough, your points are gone. If Sun costs you 60 points thats one thing, but what if it was 500?
4. Last year Microsoft was really popular - 500 points! What's it worth to you?
So you have to plan ahead a bit.... but basically its really simple.
1. You get invited. If you don't get invited you can bid.
2. If you are successful bidding you can interview.
3. If you are not sucessful you can be on a waitlist (so that if someone drops out you can fill their slot. The waitlist is ordered by bid quantity. So if the price was 50 points and you bid 49, you'll be first in the waitlist). The waitlist does move - and sometimes a LOT - especially towards the end of the recruiting cycle as waves of offers come in and suddenly 20-30 people might accept an offer.
Thats about it.
As far as people knowing you bid - in general, your interviewers don't know (and don't care) - people do get jobs from the bid lists, but I'm sure it varies by firm.