Most people are familiar with online travel sites that allow you to book hotels directly (often at cheaper rates than through a travel agent, or even if you contact the hotels directly).

So how do the online travel websites manage their accomodation stock? For example, are they somehow connected in to each hotels room management system and thereby able to see whether rooms are available?

OR

Do the hotels offer up a block of rooms to each of the online travel sites for them to exclusively sell?

If someone works in the hotel industry, perhaps they might know.