Hi chucker, most cameras these days have manual presets for white balance. My camera phone has white balance presets! While semi-professional DSLRS have presets and the option to manually select your kelvin temperature for the perfect white balance.
A grey card is used to get the correct exposure. Not
commonly
used for white balance.
A grey card is what the world would look like if you took all the colours in the world and supposedly put them into a blender, the result is 18% grey. All camera's light meters are 'set' to 18% grey.
When a camera's shutter release (that button that takes the photo) is half-pressed the camera's light meter reads the scene your about to take a photo of. If you were taking a photo of snow which is obviously lighter than grey your metered exposure would be under-exposed (it's taking the white light of snow and making it 18% grey). In this situation you would purposely over-expose unless you want your snow looking dirty.
Vice versa with scenes at night. Sounds kind of funny, under-expose (make darker) for night time shots where it's already dark and over-expose (make lighter) for things like snow which are already bright.
A lot of cameras now have smart light meters that guess that your shooting at night time and of course exposure settings for snow, fireworks, night time, etc. AND wb presets like flash, tungsten, fluorescent.
Hope the layman terms were okay
edit: In the days of film, you would use a grey card for exposure and filters for colour balance
tungsten is an orange/red tinge so you would put on a blue filter to bring the temperture back down to normal