For a Tunisian crochet stitch, using a Tunisian hook, begin by chaining as many stitches as the project needs. Insert the hook into the first chain, and pull a loop free. Place the hook through the loop, and leave the loop on the hook. Repeat until each chain loops around the hook, except for one. This is the forward pass.
To make a reverse pass, without turning the work over, loop the yarn over the hook, and pull it through the last loop on the hook. Then yarn over again, and pull the hook through two loops at a time, keeping the reaming loops on the hook. Repeat until there is one remaining loop, which starts a new row.
For the simple Tunisian crochet stitch, work down the row again, picking up one loop each time by passing the hook through the stitch. Yarn over the loop, pull through to the front, and keep the loop on the hook. Continue down the row until all the loops except one are on the hook. Next, make a reverse pass. Continue until the project is finished. You can use a regular hook for smaller projects, but a Tunisian hook helps keep the loops on the hook for larger projects.
There are several variations to the Tunisian crochet stitch. About.com and All Free Crochet both have tutorials on how to do various Tunisian crochet stitches.