Advanced organisms may be same as complex organisms. Since there is a possibility that complexity increases with time and the organisms accumalate changes for better adaptation and survival, therefore we can say that advanced organssims are complex in nature. 

Range of Variations:

(i) Life span- insects live for a few days whereas cow, dog and humans  etc. live for many years.

(ii) body structure—Organisms like cows, insects   plants and humans have different body structures. 

(iii) Size—Some organisms are  small while the others are big. Even different plants are of different sizes

The primary characteristic on which the first division of organisms is made is the organisation of nucleus. Whether organism has eukaryotic cell i.e., membrane-bound organelle, including nucleus or has prokaryotic cell i.e., has no clearly demarketed nucleus and membrane-bound organelles.

The organisms which have ancient body design that that have not changed much over the course of time, are known as primitive or lower organism.

Advanced organisms are those who have accumulated changes and acquired their particular body designs relatively recently.

The difference between the two are : 

1. Primitive organisms have simpler body design where as advanced or younger organisms have complex body design.

Plants and animals are put into different groups on the basis of whether the organisms produce their own food through process of photosynthesis (plants) or organisms get food from outside (animals).