Ravens and crows are similar in some respects but different in some others. For example, both are keenly intelligent and are believed to inhabit a realm beyond time. They have keen eyesight and can 'see' the past, the present, and the future all at once. Both have a complex language of calls and caws and shrieks that most human beings don't take time to understand. The major difference between the two animals is that crows are very social birds and live in large groups, whereas ravens are solitary and are found more often in wild, lonely places.
This difference impacts the meaning behind the two animals. Crows are associated with divine or tribal law, with connection and proper conduct within the community. Both are associated with secrecy and both warn against spilling your energy and wasting it by talking too soon, too much.
Crows warn each other and other animals, so if a crow speaks to you (and they do!) it is often a warning--either from the crow (to move along) or for you (that there is something threatening you that you need to get a clue about). I personally take crows pretty seriously, as they are very, very smart and see things we don't all the time. Some people use crows as a kind of divination--you can ask a crow a question and wait for the answer, and it will usually be right, but naturally, this takes some practice, some intuition, and the consent of the crow.
Ravens are strongly associated with magic and healing, and the realm of the dead. They are much larger than crows, and will 'play' with wind currents by rising high in the air and then letting themselves dive and whirl before pulling out of a fall. If a raven appears in your life, you might have a calling, or you may be getting advice to tune in to your own power, your own wildness.
Although this is kind of an oversimplified statement, in general crows are more about developing wisdom and power within the group, ravens with solitary introspection and personal magic and power. Both herald change, and sometimes death or the presence of the dead.
Thanks for the question!
I hope this has been helpful. I will be posting a hub on this in the future too. :)