According to ChemiCool,Affinity is the tendency of a molecule to associate with another. The affinity of a drug is its ability to bind to its biological target (receptor, enzyme, transport system, etc.) For pharmacological receptors it can be thought of as the frequency with which the drug, when brought into the proximity of a receptor by diffusion, will reside at a position of minimum free energy within the force field of that receptor.
In other words, two things that are well matched have a strong affinity. For example, organophosphates, the stuff in pesticides, have an affinity for the receptors in your brain that control your central nervous system. When living things ingest these pesticides, the chemical "plugs up" the spaces where sodium or potassium ions would normally go. These ions are carrying messages like "breathe" and "pump blood" so you can understand why having their docking stations plugged up might not be a good idea. Nicotine will bind to receptors in the brain much the same as heroin, hitting the "reward centers" of the brain. But of course, the user will become addicted once the receptors get used to having their buddy Nic around.
My friend Denny has a strong affinity for his guitar, Eric has Sylvia and poetry, Velociman has his Velocilanguage (which I can't always understand), my dear Husband loves his crime stories. And although I don't share that particular affinity, I still love him dearly. He and I have our own affinity, too, as is evidenced by our long marriage.
So, what's your affinity?