What are door chains and latches?
A door chain is a thin piece of metal chain which is attached to the door jamb. On the end of the chain is a short and specially shaped bolt, which slides into a metal track screwed into the back of the door. When the chain is slipped in, the door can only open as far as the length of the chain will permit. This allows you to open the door just a bit, so that you can more safely interact with strangers outside.
Door latches are mechanisms used to fasten a door closed. One basic example is the hook and loop latch, which is a simple piece of hooked metal, the open end of which slides into a metal loop and props the door open at that width, not allowing it to be fully opened or closed. There are many different latches with various levels of complexity involved.
How do chains and latches work?
Door chains work by preventing access from outside, even if the door is partially open. When the chain has been slipped, the door will only open as far as the short length of chain allows which is should be enough to have a conversation through, but not quite enough for a hand or arm to reach in.
Door latches may serve a variety of different purposes, but most are made to keep a door shut until they have been engaged. This is usually done by simply lifting the bolt or whatever other part of the device is keeping the door closed.
Where are chains and latches used?
Door chains typically see most use on front doors, so as to allow them to open a little to strangers. They are usually used as an additional security measure to complement an existing lock.
Door latches are common in most homes, appearing on all sorts of doors from rear screen doors to wardrobes and gates.
Are there any variations?
Door chains are almost all the same, varying only in the type of metal used and strength of the chain.
There are many different types of door latches, ranging from the simple large throw bolt commonly found on garden shed doors, to more complex cam lock arrangements for grand style entrance gates. The type of latch you choose to use for a given purpose will depend largely on whether it's being used for security reasons, for privacy, or simply to hold a door shut.