The ancient Greeks had many names for the various types of nature spirits. Nymphs were female spirits of natural formations or specific locations - Dryads were one type of Nymph, the spirits of oak trees. The Meliae were the spirits of the ash trees. Naiads ruled over streams, books, and fountains. Oceans, flowers, clouds, meadows, groves, and mountains all also had their spirits or guardians. These spirits were given various names, and were thought of and treated in many different ways by different cultures.
Whatever you want to call Them, however you wish to divide Them, They are all around. Often they neutral to humans, but some do seem to enjoy the company of humans, and others can be hostile. If you've ever read old folk tales of Fae, such as some coming from Ireland, you know they were often thought of as dangerous, and were treated with respect.
These spirits are not human, they have their own obligations, needs, and desires. They are not all the same, the spirits of the yard and those that live closer to humans can feel different than the spirits of the deep wilds - and even the spirits from one yard to another may not be the same in every way. If you wish to do spirit work, you must take the time to get to know the spirits of each area around you, rather than assuming they will all act the same, and request the same things from you - or even want anything to do with you at all.
If one wishes to know and work with the spirits, it's best to begin with a cautious and respectful approach. Seeing what traditional offerings were given in your area can be a good place to start. In the New Hampshire area that included tobacco, small stone or clay figures, beads, found feathers, and other such objects. Offerings of milk and honey, sweet grains, and wine are also commonly suggested - and while they are more European in nature, they seem to be welcomed here as well. Experiment. Leave the offerings, and spend some time outside. Take the time to listen, to really listen to these spirits that have been ignored for so long. Sometimes it can take weeks, or longer, to really begin to form a bond.
Offerings should be left whenever one takes something from the land. If you take, you give back. (Offerings aside, one should always make sure they have permission to take first.) When you leave offerings, be sure you don't leave anything that will be harmful to the local wildlife. Really think about what foods and material offerings you are leaving behind, and if they could cause harm to animals or the environment. It's often said it's the thought that counts, so actually put some thought into it.