A Tek screw (a brand name that became generic for any self-drilling screw) has a hex or button head, a hardened drill point, and self-tapping threads. The screw drills its own pilot hole and taps its own threads in one driver-driven action.
Tek screws are the standard fixing for sheet-metal and light-gauge steel work: roof and wall cladding, fence-rail-to-post connections, batten-to-rail fixings on aluminium-on-steel gate frames. Common Tek sizes are #10 (4.8 mm) and #12 (5.5 mm), in lengths from 16 to 65 mm.
Limitation: Tek screws are not structural fasteners for primary connections. Use a hex bolt or a chemical anchor where the load matters. Tek screws also corrode quickly in coastal exposure unless they're stainless or have a Class 4 or Class 5 coating.