A futures term structure is a diagram which depicts the prices of futures contracts of the same underline asset but with different delivery dates, in a certain day.

The horizontal axis of the diagram is constructed by putting on it contracts with different delivery dates starting from the most recent until the most distant. On the vertical axis we have the prices.

Below we can see the term structure of WTI crude oil futures as it was on September 28, 2012.

Notice that the further away their delivery date is, the lower their price tends to be. This situation is called backwardation and it produces a down-sloping curve. When the curve is up-sloping the market is in contango.