Messengers can be just about any being with something for us. They don't always just bring us information. Sometimes it's another kind of gift or experience, or a lesson.
1. Human Messengers:
Messengers can be people, "When the student is ready, the teacher appears", is one way we've come to describe this type of messenger. These people come into our lives for a reason, and we may have a sense of the uncanny.
2. Animal Messengers:
Other messengers can be animals appearing more frequently in our daily lives, where we don't normally see them much. Or they might be a different size than typically seen, or behaving in a way that signifies something to us.
3. Plant Messengers:
Sometimes messengers are plant spirits, as in "you are what you eat" and for example, after consuming a particular herb we may start to think differently or behave in ways that bring us to something new in our lives, or a revisiting of something we had not dealt with. Some other ways plants communicate is through meditation or interacting with the plant daily and observing it. Even an apple falling on one's head can have a message, if we believe the history books.
4. Other Messengers:
These are not the only kind of messengers out there. You might consider the Muse a messenger, or an Angel, or a dream, or a pet cat bringing home a present for you. The ways in which we receive what is for us, from the universe, are unlimited. There might be a message in just about anything if we look for the meaning and significance.