(2-part post):
See here for the official answer to your question, from the US embassy website:
http://london.usembas...sa/niv/add_crime.html
As others have indicated, it's crimes involving 'moral turpitude' which cause the most problems. The US immigration and Nationality Act states:
"Any alien convicted of, or who admits having committed, or who admits committing acts which constitute the essential elements of -
(I) a crime involving moral turpitude (other than a purely political offense) or an attempt or conspiracy to commit such a crime, or
(II) a violation of (or a conspiracy or attempt to violate) any law or regulation of a State, the United States, or a foreign country relating to a controlled substance (as defined in section 802 of title 21), is inadmissible".
The only exceptions are where the offence was committed by someone under 18 years old at the time of the offence, at least 5 years prior to the visa application; or where the maximum possible penalty for the offence does not exceed one year. (If the actual sentence passed exceeded 6 months that exception does not apply anyway).
'Moral turpitude' covers a wide range of offences. All offences of theft or fraud count as moral turpitude but minor offences of violence don't. (So, for example, nicking a Mars bar or using a child ticket for adult travel both constitute moral turpitude. Punching someone in the face doesn't). For a detailed list, see here:
http://www.state.gov/...rganization/86942.pdf