What is the difference between a single celled animal and a multi celled animal?

A single celled animal is an animal that only has one cell, Eg: amoeba, Paramecium, Euglena, Chlamydomonas. Single celled animals have one cell that does all of its life processes, such as ingestion, movement, excretion, reproduction.

A multi-celled animal is an animal that has many cells that each perform a specific function of life. These animals have a group of cells that digest, another group reporduces, another group controls movement, etc.

If you look in the Wikipedia, it offers good information as well. Hope this helps :-)