What is HomeKit?

Apple HomeKit explained

Apple's standard for Home automation

HomeKit is Apple’s standard for controlling devices in your home. It’s a standard,ot an App. You can think of it as a little database on your iPhone. Or just structured information.

Homekit contains all your devices. To begin with Apple does not call it devices but accessories. An accessory is typically something you buy in a shop, such as a light or a sensor. In order to use it in HomeKit the accessory has to be certified for HomeKit.
You can recognise Apple HomeKit accessories by this logo:
There are all kinds of HomeKit accessories: Switches, Lights, Window Coverings, Locks, Sensors, Thermostats, Air Conditioning, Fans, Bridges, Cameras, Doorbells, Faucets, Garage Doors, Air Conditioners, Air Purifiers, Humidifiers, Outlets, Receivers, Routers, Security, Speakers, Sprinklers, TVs and Windows.
Window Coverings are curtains, (venetian) blinds, roller shutters and the like.

The most well-known accessory is probably Philips Hue. Philips started to support HomeKit very early on and was to many people their first HomeKit product. And Thinka is a HomeKit accessory too.

See more about HomeKit at How To Use HomeKit?