According to Crystallinks, "Spirit Guides are entities - physical and non-physical - who have chosen to aid others on the path to spiritual enlightenment as we are about to ascend to higher light - the evolution of consciousness - our natural state of being."
They are said to accompany our souls in our lifetime or lifetimes to come. They help us in fulfilling our soul's destiny. They connect with us in a different wavelength that even our logical self wouldn't understand...being able to realize this "truth", we are easily guided to this journey where we ought to learn.
On a personal level, I believe that when we ask ourself questions...there are times that our spirit guides are the ones who answer them in a subtle way. I tend to "talk" with myself and whenever I try to decide, there's this little voice inside me...leading me to decide what to do in the end. Many times, I thought maybe it's my "intellegent" nature helped me. But I have a feeling that a higher self is guiding me through and through.
Astral Society website has a FAQ guide relating to Spirit guides that is very informative. But always remember, we don't need to have same experience to make sense of what is real or not. There will always be a aha-moment deep inside every one of us. We just need to listen to our core (universal self).