Trophoblast is the outer layer of the blastocyst. In the later stage of blastocyst, the trophoblast gets differentiated into two layers:

One layer remains cellular (because their cell walls remain intact) and is called cytotrophoblast.

The other layer becomes syncytious ( because their cell walls disintegrate and their nuclei are seen floating freely in the cytoplasm). This layer is called syncytiotrophoblast. It forms finger-like projections called the "trophoblastic villi or chorionic villi" and thus forms a part of foetal placenta.