A slip chart is simply a chart of a product's schedule slips, connecting points made by the pair of a ship date and the date it was announced.
For example, on Day 0 of the project you announce you will ship in 90 days. On day 90, you ship. Your slip chart consists of the point (0,90) and (90,90). That is, you finished exactly when you said you would up front, and your graph is a horizontal line.
Now imagine on day 0 you say you'll ship in 90 days. On day 85, you announce you'll ship on day 105. You make no further announcements, but actually ship on day 110. You'd graph the points (0,90), (85,105) and (110,105). [** shouldn't the last point be (110,110)? **] This line is nearly horizontal, with a slight correction.
In QualitySoftwareManagement, Volume 2, GeraldWeinberg discusses various SlipCharts and their meanings.
SlipCharts are especially useful, because they typically don't require any additional metric gathering by the project. That is, you can do a slip chart on the next release of Microsoft Windows just by reading back issues of PC Week, and you can do a slip chart on your own projects just by reading your press releases and internal emails.
Where I work, we call them WiggleCharts?; they are updated on a regular basis (typically weekly, based on the schedule), and they track other milestones of importance besides shipment. In addition, the negotiated deadline (between the project team and management) is tracked as well; that changes rarely (but can change if circumstances warrant)