Lancaster, Pennsylvania is probably best known for the population of Christian Anabaptist communities around it. The best known are the Amish. Lancaster City itself contains very few Amish, and an unusually large concentration of Vietnamese immigrants.
The Amish community does not particularly like visitors, tourists, or sightseers. They are an intensely private religious community doing their best to go about living according to the ways of their religion and culture, and very much wish to be left alone unless you are invited. Don't take photos without permission, or harass or otherwise pester them about their way of living. Above all else, don't trespass on private property.
There are official tourist exhibits set up by the local government and various private enterprises that will allow you to learn about the Amish in a way that is respectful and positive for everyone involved.