Glyphosate is absorbed by the leaves and works its way down to kill the plants from the roots up. This process takes up to two weeks and repeated applications may be needed on particularly stubborn plants such as bindweed and thistles.
As Geoff says, it becomes inert on contact with the soil and will only kill plants whose foliage has been sprayed or painted with it.
However, European studies in several countries have shown that it does leach out of soil into nearby water courses and this can affect aquatic plants and thus habitats. Great care must be taken when using it near water courses.