London.crafts wiki

Tunisian crochet is sort of a cross between crochet and knitting. It's generally worked on a long hook with a stitch stopper at the non-hooked end - sort of a cross between a crochet hook and a straight knitting needle. The fabric formed is a lot flatter and more even than ordinary crochet. It has a slight case of the [stocking stitch curl problem]?.

Here's an explanation (with good diagrams) of how to do the basic afghan stitch, which also explains how to stop the fabric from curling.