The passing lane is the lane on a highway, freeway, or any road that is designated for faster drivers. In most countries, this lane correlates as the opposite of the side of the road driven on - in America, the passing lane is the far left lane, and in England and other countries, the passing lane is the far right.
There are two main reasons to have a passing lane. First, it means that, despite the uniform speed limit on most roads, people can go at speeds they feel comfortable with without interfering with other drivers. Secondly, having a passing lane allows for emergency vehicles such as ambulances and police cars an unrestricted path to their destination.
In many states, it is against the law to remain in the passing lane after having passed someone. Since these laws vary, you should check with your local Department of Transportation office for rules in your area.
In conclusion, most roads with 2 lanes are made for passing on the left and driving on the right. Most roads with 3 lanes or more are made for faster on the left, slower on the right. Follow these rules, and everybody wins!