Morning sickness is a condition that 75% of pregnant women have in early pregnancy where they are nauseated (and maybe also possibly vomiting). It occurs not necessarily in the morning, though, it can happen any time of the day or night.
It usually starts between the fourth and sixth week of the pregnancy, although it has been reported as early as two weeks after conception. It takes two weeks for the fertilized egg to implant itself into the womb, and once implanted is when the dramatic hormone shifts occur in the pregnant woman, which is thought to be one of the main causes of the morning sickness symptoms. It commonly is over by the 12th week of gestation, right at the end of the first trimester. Although it can be perfectly normal to go longer into the 15th or 16th week, it is good to report to your health care professional if the morning sickness symptoms continue beyond the 13th week.
If the vomiting is unrelenting and you are unable to keep food or fluids down for a full 24 hour period or if the vomiting occurs more often than four times in a single day, it could be a condition called hyperemesis gravidarum, which requires some medical intervention, so immediately notify your obstetrician or health care professional if this happens.