SteamOS is self explanatory. It's an operating system. If you don't know what an operating system is, get back under your rock please.
Steam Machines are just some small cheap PCs Valve put together to test out SteamOS.
In the future Steam Boxes will be a thing. They will be (hopefully) small cheap PCs used to stream games from your main PC to your TV without having to have another kick ass PC, or having to install the game again on another PC. Other form of Steam Box will be just a proper gaming PC running SteamOS (Linux)
Basically, Valve have put together ideas to move PCs into the living room to compete with consoles. Few of us have had PCs in the living room for a while but this is to get them to the majority.
Im personally really excited about this. I have the parts laying around from my old HTPC. For the cost of a small case i could put it together, have it run SteamOS and play all my Steam games with the streaming feature. Doing that would allow me to sell my current HTPC, so Steam (in theory) have made me able to make some money lol.