A historic site or heritage site is an official location where pieces of political, military, cultural, or social history have been preserved due to their cultural heritage value. Historic sites are usually protected by law, and many have been recognized with the official national historic site status. A historic site may be any building, landscape, site or structure that is of local, regional, or national significance. Usually this also means the site must be at least 50 years or older.
The U.S. National Park Service defines a historic site as the "location of a significant event, a prehistoric or historic occupation or activity, or a building or structure, whether standing, ruined, or vanished, where the location itself possesses historic, cultural, or archeological value regardless of the value of any existing structure".
In the United States, the National Historic Preservation Act (which was enacted October 15, 1966 as Public Law 89-677) allows the Secretary of the Interior "to expand and maintain a national register of districts, sites, buildings, structures, and objects significant in American history, architecture, archeology, and culture, hereinafter referred to as the National Register...".  "There are over 2,600 National Historic Landmark sites in the United States, and the federal government owns fewer than 400 of them. Roughly 85% of them are owned by private citizens, organizations, corporations, tribal entities, or state or local governments — or sometimes a combination."  All of these historic sites have been approved by the U.S. National Park Service (and its special advisory board) and the Secretary of the Interior. 
Historic sites can also mark public crimes, such as Tuol Sleng Genocide Museum in Phnom Penh, Cambodia or Robben Island, South Africa. Similar to museums focused on public crimes, museums attached to memorials of public crimes often contain a history component, as is the case at the Oklahoma City National Memorial & Museum.
Historic sites and heritage sites are often maintained for members of the public to be able to visit. Visitors may come out of a sense of nostalgia for bygone eras, out of wishing to learn about their cultural heritage, or general interest in learning about the historical context of the site. Many sites offer guided tours for visitors, conducted by site staff who have been trained to offer an interpretation of life at the time the site represents. A site may also have a visitor center with more modern architecture and facilities, which serves as a gateway between the outside world and the historic site, and allows visitors to learn some of the historical aspects of the site without excessively exposing locations that may require delicate treatment.
Historic sites are important to all people and cultures. People can experience and learn what people of the past experienced and how that affected the present and continues to affect the future. Simply, it's important to preserve because it protects our cultural heritage that fulfills our need to "connect to the past while providing a framework of ideals for our future". 
For example, when Shockoe Bottom (the center of Richmond, Virginia’s slave system and economy) was proposed to be turned into a minor league baseball stadium, the community of Richmond wanted to preserve this site, even though it serves as physical symbols of systematic violence and oppression, to show that “history is reclaimed to heal and reconcile our nation’s past injustice and to honor the resilient nature of our African American ancestors”. These historic sites would be lost in time without appropriate preservation.
If you live in the United States, in order to make a place in your community a historic site, you need to know about the historic preservation ordinances in your area. Historic preservation ordinances are the primary laws that communities implement to protect and preserve historic resources. They offer the strongest legal protection for historic properties. 
Every community has different historic preservation ordinances that they use, and applicants should research their community's criteria. Applicants should prepare a "well-researched argument for the commission to review at a public hearing, where they will give their recommendations and/or approval for designations."  If someone feels that an important historic location is being threatened, then applying for it to be a historic site would be worth looking into. Remember if the location is privately owned, the owner has the right to accept or reject the proposal.  Federal funding through grants can assist with the maintenance, but the owner is under no obligation to comply with the National Park Service's recommendations on how to preserve the property if no funding has been granted.