From Wikipedia.org

The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula, but before implantation. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled blastocyst cavity known as the blastocoele. The human blastocyst comprises 70-100 cells.

Blastocyst formation begins at day 5 after fertilization in humans, when the blastocoele opens up in the morula.

Differential gene expression in the morula is thought to be the cause of the lineage divergence of different cell types. For example, the Oct-3/4 transcription factor is restricted to the ICM, whereas Cdx2 is expressed at a higher level in the trophoblast than the ICM. This differential transcription factor expression is likely the result of positional effect - cells in the middle of the preceding zygote are in a different environment to those on the outside, thus causing differential expression.