Best Answer: In order of appearance:
Chromatin is the uncondensed DNA that is present in the cell during interphase (the cell's normal "growth and development" stage). Think of this as unraveled DNA that is laying around in the nucleus. Because it is unraveled, transcription machinery can get to it and use it to transcribe mRNA. mRNA will then leave the nucleus and go on to help in making proteins.
Chromosomes are super condensed DNA that is present in the cell during mitosis (cell division). Imagine if all that chromatin "packed up" and wrapped itself nice and neat, kind of like when we wrap up electrical cords of adapters or appliances. Chromosomes are structures of highly condensed DNA that make it easy for the cell to divide and distribute its DNA into the two new daughter cells that will be formed from the existing cell. Chromosomes can contain one or two CHROMATIDS, which I'll talk about next.
Chromatids are one half of a mitotic chromosome. You know how most chromosomes look like X's? Well, one half of that "X", the ">" half, is a chromatid. Chromatids are usually identical. Why does a chromosome have two, then? One chromatid will be taken into one of the new daughter cells in mitosis, and the other chromatid will be taken to the other. The cell itself has machinery that literally rip apart chromosomes at their center, letting each new cell take one chromatid with it.