Preclearance, also commonly known as pre-flight inspection, is offered by US Customs and Border Protection (CBP) at select locations around the world and allows US-bound passengers to undertake all immigration, customs, and agricultural inspections prior to boarding the flight outside the US.  

How is preclearance different than the “normal” post-flight inspection upon arrival in the US?
A preclearance inspection is essentially the same inspection that travelers would undergo at a US port of entry. The only difference is that it is conducted outside the US. The advantage is that preclearance provides for domestic-style arrivals at US airports. Generally no additional immigration/customs forms will need to be completed upon arrival in the US.

Where are preclearance sites located?
There are many preclearance locations all around the world in such countries as Canada, Bahamas, Bermuda, Aruba, Ireland, and the United Arab Emirates (and possibly one day in the UK). These locations are based on agreements between the US and the host country, which allows CBP to staff officers at host airports and facilitate the customs and immigration process for passengers prior to arrival in the US.