Dreaming is a very important function of being human. The realm of dreams is where our inner and outer realities meet. It’s where we can find insight and guidance.
I think that there are two basic types of dreams: process dreams and prophetic dreams. “Process dreams” are those weird jumbled messes of bizarre imagery and twisted plots that comprise the bulk of our normal dreaming. It's the mind's way of sorting through all of the information we've gathered through the day. These dreams are rarely of significance, other than as a reflection of where you've been. These dreams fade away like the morning mist at sunrise. You don't remember most of them, and the few you can recall upon waking are gone by lunchtime.
The “prophetic dreams”, on the other hand, usually have an impact on you. They are much more vivid, and you remember parts or all of the dream clearly, even hours after waking. They might be just as bizarre, but they are important. These are the dreams that are trying to tell you something. They are called prophetic because they usually refer to the future either directly, or reflect current tendencies that have future repercussions. For example, you might dream of dead fish washing up on an unknown beach, and upon waking, realize that you haven't fed your goldfish in the past few days. That's a prophetic dream.
Most people dream symbolically, and dream symbolism is as individualistic as it comes. Your mind can create images for you that are dredged up from the very depths of your unconscious. These dreams are sent to teach you things that you have been ignoring or neglecting in your waking life. They say: "Pay attention! You've missed something!".