AD or Offer ??

I received a letter from a school weeks ago: "On behalf on department of mathematics, I am pleased to report that you have been granted admission starting with the fall 2010 semester. You are also being considered for a teaching assistantship. It will take us a few weeks to determine how many TA positions will be available. We will be contacting you about this possibility soon." This letter was sent on February 19th. Is this an offer or AD ?? If this is AD, do I need to talk with the chair of department or contact my potential advisor ?? Thank you very much.