I've always found the concept of countertransference difficult for some reason. The basic definition is pretty clear: countertransference is the expression of the therapist's own experiences, thought, beliefs, emotions, drive, etc. in the therapy session due to something having to do with the client. This could include what they say, how they act, how they look, or really anything that has the ability to set off a reaction in someone else. I think that normally this coutnertransference is seen as negative - mostly due to the fact that many therapists do not identify their own countertransference and act on it, much to the detriment of one party or the other (or the relationship itself). But it doesn't have to be negative, especially if that person can recognize their reaction for what it is and use it positively.
This is not he reason why I write this, though. I have found countertransference difficult in the past because it is difficult to use in metaphor. I think that this is an odd statement and do not really know how to put it any other way. I have come up with a metaphor that really helps me to describe it now. I look at it like every person omits a "noise" - some kind of psychic communication that has to do with everything that has ever happened to them (sort of like the sun emits light or people emit pheromones). These rays of communication push out and interact with the rays of others. Perhaps an even better image would be spherical constructs that are around everyone's head.
When a client's own sphere engages or interacts with someone else's - meaning that their own communication, style, look, etc, has overlapped - certain constructs can clash. For example, a client is coming to a session because of a divorce. They communicate that want. The therapists's own sphere shows that he or she is going through a significant rough patch with their significant other. These two spheres rub each other the wrong way, creating friction, which can inform the thoughts/emotions/reactions of therapist. Frequently this friction occurs in the client and is pushed back toward the therapist (transference). Then the therapist, in order to push this negatively-charged psychic energy away, converts it into verbal or physical communication. This can be pushed back onto the client. When it is, a clash normally ensues and the client might leave the relationship.
As mentioned, when friction occurs, an experienced therapist can identify it and use it to further positively treat the client. In fact, going on with the metaphor, the energy created through the friction of the spheres can be use as a type of energy that recharges the therapist and/or the relationship; this might be use for fodder for good conversation with the correct client.