They are committed to forming certain types of cell, but not others. Restriction of potential and commitment are two features of the process of differentiation. Although differentiation is a gradual developmental process, it is still discontinuous. That is, it proceeds in steps, but they are often very tiny ones. There is nothing different about the process of mitosis, but the daughter cells are different from the parent cell in that they express a different subset of genes.
Cellular interactions trigger stable changes in patterns of gene expression.
Mostly these interactions are between different tissues, i.e. differentiated cell groups. So differentiation begets more differentiation. These interactions can be either instructive or permissive . One tissue can instruct another to change - the change won't occur if it doesn't, but will occur if it does, even artificially or inappropriately. On the other hand a tissue may only help create the environment that will permit its target cells to change, with other tissues also being important.