If you had to guess who is Google's biggest competitor, would you say Amazon? Nah, you'd probably say Microsoft because of Bing or Apple because of iOS, or Yahoo because of display ads but nope - Eric Schmidt said Amazon.
Eric explained:
But, really, our biggest search competitor is Amazon. People don’t think of Amazon as search, but if you are looking for something to buy, you are more often than not looking for it on Amazon...
Also, Amazon is huge into cloud servers and Google is also big in that area. Both are encroaching on each other businesses on some level. Delivery, music, mobile phones, search, shopping, etc.
Apple clearly isn't focused that much on search, outside of Siri. Microsoft maybe is just too slow to scare Google? But it may be someone else they aren't aware of yet that may come out of no where, as Eric said in the interview.
Forum discussion at WebmasterWorld.