There are three classes of therapist emotional response within the therapy setting:
1) Affective resonance. Any affect experienced by the patient (or anybody else, for that matter) is displayed on the face and through alterations of voice, posture, and odor. This makes the one first experiencing the affect into a broadcaster of affect, and the other person in the interaction into a receiver as s/he mimics the display of the broadcaster and experiences a valid but lesser degree of that same affect. This affective resonance is the biological precursor to mature empathy, and accounts for the ways lovers and other people used to interpersonal intimacy seem each to know how the other feels. Nothing pathological here---this is normal and a significant part of interpersonal life.
2) Transference: Even though we pretend that we know each other pretty well, the reality is that individuals only know the surface and precious little about the inner world of that other. Our life experience leads us to draw conclusions about people not from the new data they supply us, but from information assembled in the past from these other relationships. Inevitably, then, we tend to exhibit affective reactions to others that are based not on the real and true nature of the other person, but on our own history-based preconceptions. Harry Stack Sullivan called these reactions "parataxic distortions" (as if our judgement had been "drawn aside" from the real other), while Freud gave the term "transference" to such emotional constellations when they take place during a therapeutic encounter. Thus, the patient's inappropriate emotional reaction to the therapist is called a transference reaction, and it can be very useful in therapy when traced to its source in the patient's history. Nevertheless, the same sort of inappropriate emotionality may be experienced and expressed by either the patient or the therapist, and it is still technically a transference reaction. As therapist, I am as capable of distorting my patient as the patient is capable of distorting me. Actually, the errors in therapy devolving from this latter situation form one of the major reasons we like to know that our therapist has had a significant therapeutic experience.
3) Countertransference. This is the highly specific situation in which the therapist forms an emotional reaction to the affect expressed by the patient; it is different from affective resonance in that the reaction of the therapist usually turns out to be analogous to the way a significant past figure (mother, sibling, father, etc.) had reacted to the patient in the past. If the therapist reacts from his/her own background, this is another form of transference (it devolves from the therapist's real self). Countertransference, therefore, is an affective counter to the transference being experienced by the patient, and can provide excellent data about the interpersonal situation of the patient back in the days when the (archaic) transference was formed.
In sum, then: affective resonance is normal and basic to competent interpersonal function; transference is normal unless one happens to be a Zen Master who has no misconceptions about anything; countertransference is the way we respond to the other guy's transference. In all of these situations, the therapist experiences unexpected emotion as the result of the interaction with the patient.