Vomiting in cats can be a minor condition (eating something new --a new pet food, disagreeable or too fast) or something major. Any time the vomiting lasts for a over a day, is bloody or your cat is not eating or drinking regularly, see your vet.
Hairballs can be the problem (see hairball info on this website) which can be serious if it is blocking the digestive system. Or, it can be a bacterial infection, kidney and liver problems, pancreatitis, diabetes, hyperthyroidism, antibiotics, and a number of other conditions.
If you would like to explore graphic details of what can be causing the vomiting, a link that is helpful is : http://www.vetmed.wsu.edu/clientED/vomiting.aspx