Self tapping screws come in a number of different sizes and threads. A self-tapping screw is Self Tapping Screw - Countersunk Head - A2 Stainless Steel - Box, 1 1/2a screw that can tap its own hole as it is driven into the material you need fixing.
We have Zinc plated self tapping screws and also stainless steel. Self-tapping thread screws (and bolts) have a specifically designed thread pitch which enables them to self-thread into a substrate after being inserted into a pre-drilled and slightly smaller pilot hole.