Answer: Cooking with gelatine will come up in a lot of recipes because it is a thickening agent made from either collagen (a protein found in animal connective tissue and bone) or certain algae (known as agar-agar).
It is found in some gummy lollies, marshmallows, jelly desserts, and some low-fat yoghurt.
There's not really much of a difference between the two, it really comes down to personal preference.
We tend to use dried (powdered) gelatine in our recipes, simply because it's readily available at all supermarkets.
Leaf gelatine can be interchanged with powdered gelatine — 3 teapsoons of powdered gelatine (8g/1 sachet) is roughly equivalent to four gelatine leaves.
Professionals use leaf gelatine because it generally results in a smooth, clearer consistency — it's perfect in dishes where appearance really counts.
The only drawback of leaf gelatine is in sourcing it — it's usually not available from supermarkets, but can be found in specialist cooking shops.