The VC-25A replaced the VC-137C (a military version of the Boeing 707) as the mainstay of the Air Force One fleet. On some occasions, the VC-25s serve as transport for the US vice president, for which service they use the Air Force Two call sign. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.