Some leaves are poisonous, and some contain high levels of oxalates, an antinutrient that blocks absorption of minerals and causes kidney stones. In general, the leaves of trees and bushes are not edible, but some weeds do have edible leaves. Some wild plants have leaves that are edible, but only after boiling in multiple changes of water.
The most recognizable of the edible wild plants is the dandelion. It's better to pick the leaves before the plant has flowered, because they become bitter and more toxic after that. The same is true of all greens - they must be picked before they "bolt."
Two other common weeds that are quite edible, and have been eaten by humans in the past, are lamb's quarters and chickweed. Purslane is also a weed, but it has become popular as a salad vegetable recently because it contains omega-3 fatty acids.
Some herbs grow in the wild, such as mint. Basil is a member of the mint family, as is shiso, also known as perilla or beefsteak leaf. Shiso grows all over the place here in Philadelphia. It is used to make pickled plums, a Korean and Japanese favorite. In the West, grape leaves have been used to keep cucumbers crispy when they are pickled, and of course pickled grape leaves make a great wrap for Greek dolmas. I don't recommend eating unpickled grape leaves, though.
There are many books on botany, but one book that taught me a lot about edible wild plants (and pickling!) was "The Revolution Will Not Be Microwaved" by Sandor Katz. Susun Weed also has considerable expertise on the subject.
