Good question! There are 2 very different types of spirit guide references, although at "heart", they are both different sides of the same coin.
The first common reference is the ethereal guide that many spiritual mediums use to help them get information during a reading. Some of the most famous mediums claimed that they got much of their information from those living in spirit, but who had once lived on earth (although not all mediums use spirit guides, certainly some of the most famous do and have).
The other common spirit guide reference is the idea of an "invisible helper" that each of us has around us at all times, whether we know it, or even believe it, or not. Most people who study the afterlife, and psychic abilities believe that our loved ones are ALWAYS close, and that those in spirit DO get involved in helping give our lives here a bit of direction and purpose when they can.
Are spirit guides "all knowing" or are they just like us...only on the other side?
This is actually a great question, and one that comes up quite frequently. The truth is, in my own experience, spirit guides are just like us, only they've crossed over to the other side and have gained or garnered some additional knowledge as a result of having their awareness expanded (of course, just knowing that death is NOT the end allows you to be more "enlightened" than we are here).
That said, I truly believe that there are many, many levels of learning and that just because someone has crossed over, and is "in spirit", it doesn't mean they know everything, or have access to all of the answers (much the way as we age here..we simply know more, and have more experience, but that doesn't mean we know everything by a LONG shot).
How can I access my own spirit guides?
Easy. First acknowledge that they are out there.
Next, be proactive. Meditation and visualization can be very powerful. Asking for a sign or symbol can be a very powerful as well...and when it comes, it can be a shock, too! (but in a good way).
Speaking to psychics and mediums who have the gift is a great way of getting validation that you have spirit guides on the other side, identifying who they are (often this can surprise you!) and getting first hand evidence and proof that they really ARE "out there" and watching you grow.
Listening to your own intuition is one of the most powerful ways to tap into the abundant energy that exists in the Universe around us at ALL time...even if we aren't aware that it is. As a matter of fact, it's rare that a spirit guide or guardian angel will appear in your life, unless you proactively open yourself UP to the possibility.
Like everything else in life, discovering the truth about spirit is about opening yourself up to the magic and the mystery...and then seeking out experiences that give you the first hand PROOF that they are real. No amount of articles like this one can substitute for ONE empirical experience with a genuine psychic or medium, and that's the most important lesson of them all.