In most of the animal population, an embryo is an organism in the earliest stages of development, from fertilization until birth.
In Humans, the term Embryo is used predominantly in the first week of development until the eighth week, after which, the term foetus is most commonly used. 
Development of the embryo is called embryogenesis, and begins after the zygote has been formed upon fertilisation of the egg by the sperm. An embryo contaians a full compliment of genetic material, but has little in the way of recognisable external features. once these begin to develop around the 9th week of pregnancy, the embryo starts to be refered to as a foetus .
The first divisions of the fertilised embryo produce cells known as embryonic stem cells, which are totally unspecialised, also known as pluripotent. These cells have the potential to be used to treat a vast number of diseases, however ethical concerns have prevented the full extent of this research to occur.