  We introduce the notions of infinitesimal extension and square-zero extension
in the context of simplicial commutatie algebras. We next investigate their
mutual relationship and we show that the Postnikov tower of a simplicial
commutative algebra is built out of square-zero extensions. We conclude the
notes with two applications: we give connectivity estimates for the cotangent
complex and we show how obstructions can be seen as deformations over
simplicial rings.
