The timing of vomiting and the nature of the vomitus can be indicators of where the problem lies.
Immediately after eating: often due to a stimulus in the esophagus. Acid reflux could be indicated, though there are other possible causes. If the vomitus contained food it would be virtually as-eaten, possibly with clear or frothy digestive juices.
A short interval after eating (1/2 - 1 hr): Undigested or partially digested food, especially with bile, would suggest an origin in the stomach. Possible causes could be related to irritation or overacidity. One source of irritation might be antibiotics or other meds. In some cases adjusting the dosage or switching to an alternate medication might solve the problem. In Coco's case, a standard dose of Clavamox (calculated for a cat up to 12 lbs) would irritate her stomach, but she could tolerate a dose calculated for her actual weight which was closer to 6 lbs. She was also ok when Clavamox was replaced by amoxycillin, which lacks the strong acid. Ferrous sulfate as an iron supplement can be very corrosive to the stomach, whereas organically-derived sources are more innocuous.
After more than an hour: might originate in the intestine. Food may have been digested to a uniform brown liquid containing bile, or it might appear as a formed stool-like mass (though hair-balls may also take this form). Serious possibilities needing further investigation could include IBD (inflammatory bowel disease) or neoplasia. Since invasive means such as a biopsy are necessary to confirm these diagnoses it would be best to attempt to rule out and /or treat other possibilities first.
After much more than two hours: the food might not be moving through the GI tract at a normal rate. If this is the case your vet may prescribe cisapride, a powerful drug that stimulates the rhythmic contractions necessary to keep things going in the right direction. It's so strong that it may be virtually impossible for a cat receiving it to vomit. It's possible that the same clinical symptoms could be caused by a blockage, however, so additional tests such as x-rays or at least an expert abdominal exam may be advisable before its use. This type of drug could be dangerous or even fatal if the GI tract is blocked.
Often a cat will make a characteristic vocalization prior to vomiting. If you get such a warning try to put him over a newspaper when he's about to throw up. Stroke rhythmically from the shoulders back, speaking in a reassuring voice. If it's just heartburn or acid it may help relax him and avoid the episode. Otherwise it can make the experience go more smoothly and less traumatically. It lets your kitty know that you care and want to help. If you can catch it on the newspaper it gives you a good background to see what came out, and to preserve a sample to show the vet if needed.
Recurring vomiting episodes are significant and if they become a regular event it would be good to keep track of, even if just in a simple log on a sheet of paper. This will help you remember the details and give a clear picture for your vet to evaluate.
For a list of things to make note of, see Vomiting Assessment Checklist
In fact, it's a good idea to keep a log of everything related to your kitty. WE are the primary caregivers for our furry family members, and we should keep AT LEAST as good a chart as the vets that don't see them from day to day. Having good records enables you to bring your vet up to date. The vet's chart for your kitty is probably several (if not dozens) of pages long and nobody could possibly remember or read it all while you're on the phone. Your obvious involvement is also likely to inspire your vet to suggest treatments and alternatives rather than taking the easy road to euthanasia as an early recourse.