It seems to me that some people here claim that it is easy to get a September letter. It's not easy to get one given the current political context in the US. Don't be fooled from people promising a free pass.
The current regime is hard on natcotics and sex crimes. The US is not as open as canadian society is when it comes to drug liberalization and prostitution. Some states are more progressive but it's the US federal laws that take precedence at the border.
The only way someone can get a September letter is if the crime she/he committed had a maximum punishment of six months in jail. The crime in question is a CIMT (crime of moral turpitude). Drugs (including possession of a joint or weed) and prostitution/solicitation are exempt from this rule and someone will always need a waiver. Solicitation however can good you a September letter in some rare occasions.
DUIs and simple assaults (crime of passion) are not CIMTs according to US immigration law. However DUI/DWI while on drugs do count as a CIMT. However, multiple general assaults and DUIs (alcohol related) that resulted in more than 5 years of imprisonment do require a waiver.
Committing a crime while under the age of 18 counts as juvenile delinquency and therefore does not require a waiver. However, there is another constraint. The maximum sentence you can get is no more than 5 years and must be paroled by age 23.
There is a list of what constitutes a CIMT or not available on the Internet (official US immigration website - INA). Many here will claim that they can get you a September letter. It is a hard process and your packet must prove to DHS that your crime was not a CIMT, which is difficult to do. The concept of CIMT is also quite vague and open to interpretation.
Conditional discharges do count as a conviction for US immigration. Absolute discharges and peace bonds do not. Unfortunately, under US immigration law admitting your guilt to a CBP officer about committing a crime without ever being caught will result in a lifetime of waivers. Therefore, people with absolute discharges, peace bonds and charges that were stayed, dropped or resulted in acquittals may result in refusal to entering the US.
Once you are charged or arrested, your fingerprints are stored in the RCMP criminal database. You need to ask the arresting police force to destroy the record once you are acquitted or discharged. This can follow you for a lifetime.