Aspiration pneumonia is what happens when you get oropharyngeal substances, colonised by bacteria, into the lungs.
In other words, the principle problem in aspiration pneumonia is the bacteria that are suddenly deposited, free of charge, into the lung. Gram positive and Gram negative aerobic bacteria are the commonest isolates, with anaerobes occurring more rarely. Risk factors for this unfortunate case are primarily dysphagia (e.g. following a stroke) and gastroesophageal disorders.
Aspiration pneumonitis, on the other hand, is what happens when you get sterile, but highly acidic, stomach contents into the lungs. Aspiration pneumonitis is sometimes known as Mendelsohn syndrome.
Aspiration pneumonitis correlates well with a decreased level of consciousness. The damage is caused primarily by direct damage (and consequential inflammatory response) to the lung from the simple acidity of the aspirated contents. Infection is not usually a consideration, at least initially, since the stomach is normally sterile, and thus antibiotics aren't indicated. Exceptions to the latter rule would be a stomach of decreased acidity, thanks to such things as proton pump inhibitors and antacids, or when the stomach contains particulate material.