The word "affinity" is used in a sentence as either a noun or, more rarely, an adjective. An example of "affinity" used as a noun is found in the sentence "She has an affinity for the color blue."
In the above sentence, "affinity" means a natural liking for, or an attraction to, a particular thing. In science, the attraction can literally be a force holding two atoms together, as in the sentence "Hydrogen and oxygen have a strong affinity for one another."
As an adjective, "affinity" means pertaining to people sharing similar interests. An example of this use is "The planning meeting was for the drama club and all affinity organizations."