Door Knockers

Do I need a surface-fixed or bolt-through door knocker?
Solid wood doors can use both surface fixed and bolt-through products. Most types of doors can use bolt-through systems, although they may need to be fitted professionally, but for non-wood doors (plastic, composite etc.) you'll need to check with your manufacturer or joiner.
What's the difference between surface fixing and bolt-through fixing?
Surface fixed products (usually door knockers) are screwed (fixed) to the exterior surface of a door. These products are supplied with wood screws which makes them ideal for use on timber doors. Bolt-through products are bolt-fixed to your door. This requires a hole to be drilled all the way through the door so the bolt can be secured by a nut on the inside. 
What's a strike plate?
Strike plates are designed to protect the surface of your door where a knocker would otherwise hit it. It's a small piece of metal that's usually fitted the same way as the knocker itself, i.e., surface fixed or bolted-through.