There is is normally used when something is talked about for the first time and then It (or They) is used to refer back to the original thing or situation.
e.g. There is a ship over there. It has come from Denmark.
e.g. There were 100 people at the conference. They came from all over the world.
There is is also used for something that exists or is located somewhere.
e.g. There is a lot of traffic during peak hour.
e.g. There are ATM machines all over the city.
It is is usually used in statements about the following:
fact – It is our opinion that all children should have ten years of education.
time – It is half past eleven.
weather – It is raining now.
distance – It is not far to drive to Drammen from Oslo.
general environment – It is very dusty in this house.