Yes, it is almost an acceptance. They only check if you meet the minimal requirements (GPA, TOEFL if you're international.....and they don't even check the GRE, because the Grad College does not have such a requirement).
If all these requirements are met (which they should be, if the department did its selection properly), you will get an official acceptance along with you student ID. Sometimes this acceptance is subject to the receipt of certain documents, e.g. your final, official transcript if you're still working on a degree right now.
Basically, you're in. Congratulations!
BTW it took only 3 weeks, not 4-6, in my case.