I have never met a baby that didn't on occasion spit-up. Many perfectly healthy babies can even spit-up quite a bit. Reflux is often the label given to babies who vomit, and this rarely amounts to a significant problem.
However, there are a few things that a parent should watch out for:
The most important thing is the color of what a baby is throwing up. Dark yellow and especially green vomit is never normal in a baby and demands immediate medical evaluation as this could represent a dangerous twisting of the intestines (midgut volvulus), which is linked to abnormally positioned intestines (intestinal malrotation).
Another consideration is quantity. If a baby is throwing up more than 1/4 of what s/he is taking in, vomiting to the point of possible dehydration (fewer wet diapers), or vomiting for distance (projectile); then this may suggest a problem called pyloric stenosis. Pyloric stenosis usually affects babies less than 2 months of age and progresses from a little spit-up to seemingly their entire feeds coming back up in a sometimes impressive fashion shooting out (projectile) of their mouth. This results from the rapid growth (hypertrophy) of a muscle called the pylorus which surrounds the exit of the stomach. This overgrown muscle squeezes closed the exit, so the only option for milk in the stomach is to go back the way it came. Pyloric stenosis affects about 1 in 400 babies, and it is often diagnosed with the help of ultrasound. Pyloric stenosis is treated very effectively by a surgery which spreads open the muscle to allow passage of food out of the stomach. This can be accomplished by a minimally invasive approach called a laparoscopic pyloromyotomy.
So remember: beware the baby that vomits green, and it is not normal for a baby to throw up for distance!