When they list the income of someone like Steve Balmer ($1,267,670) in wikipedia, is it the monthly or yearly income?
That is typically yearly income. In Balmer's case Microsoft is a public company and his salary and bonuses are listed in MSFT's 2010 proxy.
In the US, salary discussions are almost always annual. Unless you're talking part time work with varying hours, then hourly makes more sense.
The income listed are usually yearly salary.