Theory:

The organisms that are made up of more cells are called multicellular organisms.
These cells group together to form different tissues. Tissues forms organs and organs give rise to an organ system which performs specialized functions.
Example:
 Fungi, plants, birds, insects, humans and animals.
BeFunky-collage (13).jpg
 Left to right: a mushroom (fungi), plants, bird and insects
 
Difference between unicellular and multicellular organisms
Unicellular
Multicellular
The organisms made up of a single cell to carry out all the function.
The organisms made up of a large number of cells.
There is no division of labour. A single cell performs all the activities.
There is division of labour. Cells are specialized to perform various functions.
The life span of the organism is too short.
The life span is prolonged.
Reference:
https://upload.wikimedia.org/wikipedia/commons/c/c2/Amanita_muscaria_%28fly_agaric%29.JPG