my grandfather had children with another woman and that woman has children too. what are they to me?

theyre your family

absolutely nothing..... your grandpa's wife's kids....

Your fourth cousins twice removed sideways.  

Your grandfather's children would be your Aunts and Uncles.  They would be half-siblings to any of his other children - which would include which ever one of your parents is his child.

The woman's other children would be nothing to you.

If your grandfather actually married this other woman - she would be your step-grandmother and her children would be your step Aunts and Uncles.  

Nothing.........