used to identify a specific person or thing close at hand or being indicated or experienced.
don't listen to this guy
referring to a specific thing or situation just mentioned.
there was a court case resulting from this incident
used with periods of time related to the present.
I thought you were busy all this week
used (chiefly in narrative) to refer to a person or thing previously unspecified.
I turned around, and there was this big mummy standing next to us!