Yes you can use Grey water from sinks, washing machines, showers etc to water your plants and it's a good way to save money on your water bill since you would not be sending that grey water down the drain to the sewer plant and you can water your plants with it as well.
After taking a bath I will take a couple of buckets and scoop out a couple buckets of the bath water and then take it out and water my garden with.
The water gets naturally filtered and recycled that way and then I don't have to use more water to water the plants which keeps my water bill down.
You can also run the drain pipes into a grey water tank in the ground and then add a sump pump that will automatically kick on to water your garden with too.
A friend of mine that lives in a rural area does that as she has her sinks and shower drains as well as her washing machine drain into some barrels she buried in the ground and then as they fill up it turns the pump on to water the garden with.
She also sometimes uses it to water her grass with as well.