What is actually happening when a person swallows food and feels discomfort? Growing up, we would say, "It's going down the wrong tube."

B.

The medical term for "going down the wrong tube" is aspiration. We have a common tube (the pharynx, or throat) for eating and breathing. At the level of the vocal cords, this common pathway divides into the trachea (windpipe) and esophagus (swallowing tube). If food or drink goes into the trachea, the unfortunate victim coughs to try to clear the material from the airway. While this may be painful, pain is usually not the dominant symptom -- cough is the most obvious symptom. Of course, if the aspirated material is big enough (a chunk of prime rib, for example), then the victim may not be able to breathe, may not even be able to cough. If there's no one handy to apply an expeditious Heimlich maneuver, the victim may choke to death.
No, if discomfort is the primary symptom, I doubt that we are dealing with aspiration. Pain suggests a problem in the pharynx or esophagus. What sort of problem? Here's a partial list of possibilities:

1. Infection or inflammation of the pharynx or esophagus. The common "sore throat" can be due to viral, bacterial or even fungal infection. (Thrush is an example of a fungal infection.) Sore throat can also be due to acute or chronic exposure to irritants such as alcohol, cigarette smoke or gastric contents (as in patients with gastroesophageal reflux disease, or GERD). Rarely, pain with swallowing -- the medical term is odynophagia -- may be due to a deep neck abscess. This is a potentially life-threatening infection.
   
2. Tumors of the pharynx or esophagus. Such tumors commonly present with odynophagia and dysphagia (difficulty swallowing; the food "gets stuck"). Tumor is an uncommon explanation for these symptoms, but is obviously a worrisome possibility, especially if the tumor is malignant. Risk factors for cancer include tobacco use, alcohol use and advancing age.
   
3. Motility disorders. "Motility," in this context, refers to the ability of food or drink to move down the esophagus. Gravity helps, but gravity is neither necessary nor sufficient for an effective swallow. What IS necessary is a sequence of highly coordinated muscular contractions of the esophagus. Many conditions can alter esophageal motility. In some disorders, the esophageal muscles contract forcefully (spasm) in response to swallowed food or drink. This is often very painful. Esophageal spasm is treatable, but it must first be correctly diagnosed. Diagnosis of esophageal motility problems involves a careful history and physical examination by an appropriately trained physician (such as a gastroenterologist), followed by tests (such as an esophagram, or "barium swallow," and esophageal manometry, in which a device measures pressures at various points within the esophagus).
   

In summary, odynophagia has a variety of possible explanations, some of them quite serious, all of them treatable. If this symptom occurs with any regularity, you ought to discuss it with your doctor.