To understand what a "walk-in" is one first needs to understand that our physical body is not *us*. It is the container for *us*. Similar to someone driving a car. When we see the driver we understand that they exist irrespective of whether they have a car, or not. So too with our bodies. We exist irrespective of whether we have a physical body or not. In other words we existed as a spirit (soul) before the body we now occupy was born. When our physical body dies we will continue to exist without it.
Now most people would understand that each physical body is owned by someone. This person is attached to their body (with a non physical cord) until the body dies. If nobody was attached to the body then it would die. This is one of the reasons babies are born dead. Nobody has become "attached" to the body. Without this connection the body gets no energisation from the spirit realms. Like a car without petrol or battery. It probably won't do anything. Generally then it is one body one spirit. Multiple people can inhabit the one physical form and this may be termed possession, obsession, mutiple personality disorder or whatever. However only one spirit is attached to the body. Until physical death of the body. There is however one exception to this rule. Walk-ins.
Walk-ins are people who have spiritual work to do in the material world but for various reasons don't need to/want to spend the time in a physical body growing up. Where a person has finished their life's lessons, and would normally die, in some instances they allow an incoming "walk-in" spirit to be connected to their physical body (that they are leaving behind) instead of them. We still have the case of one physical body, and one spirit attached to it, but in this situation the spirit isn't the one that was with it since birth. Naturally this can then mean that the person may seem to have a (sometimes major) personality change. This is in fact one of the ways one can tell if someone is a walk-in. They will be very much more compassionate, loving, spiritual than before.
This can of course create problems for friends, and relatives. They often cannot understand what has happened. In this situation it may be better for them to understand that the person that they previously knew is, in practical terms, no longer physically alive. It is sometimes not much easier for the walk-in. He/she inherits the memories/situations of the previous occupant of the body and may take some time to understand their true status. Walk-ins have always existed but due to the changing of the consciousness of the world at the moment a much larger number of them are incoming at the moment. The purpose of this is to help the planet move forward to a higher spiritual perspective. Instant incarnation, as an adult, is a very effective way to get a lot of people into the physical world, helping, in a very short time.
If you want to know more about walk-ins then you would do well to read the books written by Ruth Montgomery. The relevant ones are called "Strangers Among Us" and "Threshold To Tomorrow". They are currently Fawcett Crest titles being published by Ballantine Books.