There are intra-contract spreads, as Arabian described. They are popular in STIRS partly because of the liquidity (lol), and partly because they are about maths; the Dec 09 Dec 10 contracts are not just correlated, but thanks to Mar10, Jun10 and Sep 10, it can't get too far out of line with "fair" value without being arbed away.
If you are talkin about the Oil markets, then you still have intra contract trades (like backwardation / contango positions, but as I understand it you really need to be able to trade the cash and basis to make these work properly), but also the inter-contracts (crack spread, for example).
Spread trading generally works in two ways; either taking relative value positions (in which case it is very much like pairs trading, either inter or intra contract), or building an inventory of contracts you collect an inside market prices - an inventory that pretty much sums to nil, so you have very little, if any, exposure to directional movements - and offloading the contracts to a price taker, and earning the spread
others will be able to add more