Organic weed killer or herbicide can be used in organic gardens to kill unwanted plant growth without introducing dangerous chemicals into your backyard. Many people will look for organic weed killer even if they do not have a 100% organic garden. In a vegetable garden, you probably do not want to spray a lot of unknown chemicals onto plants that you are growing for food. And even if you only have lawn and flowers, organic weed killer will usually be safer for pets. Do check labels, however, because ingredients can vary.
The word 'organic' has two different meanings. One is any substance that is carbon based. This includes virtually all weed killers. The other meaning is a product that can be used on land that has been certified for organic food production. This is what we usually mean when we talk about organic weed killer, and there are not so many of these products. So if you are buying a commercial weed killer, you will need to check that it is for organic land.
Herbicide (which just means 'plant killer') comes in two types. Selective herbicides kill certain plants only, for example, a broad leaf weed killer will kill clover, dandelion and ...