Another way to look at attachment (from a developmental perspective, which also applies very much to therapeutic relationships) is that attachment is the deep emotional bond that develops between a child (client) and a caretaker. In order to form secure attachments conditions have to be met on both sides. The child has to be able to communicate a need, the caretaker must be able to understand and respond to the communication, and the child must be able to be soothed by the response. If these conditions are not met, then attachment will be insecure.

Transference is about taking something from one place or setting and moving it to another one. In therapy from some orientations it is assumed that any feelings for the therapist are transference because something about the interaction with the therapist is similar to an interaction you have had someplace else, so with the therapist you replay it and can work through the feelings and learn about them and use them to understand yourself.

So having a crush on your therapist could have to do with attachment and transference, and one of the more productive ways to handle that situation is to explore those possibilities with your therapist and see if together you can identify the feelings and how they might relate to your life outside of therapy.