Multicellular organisms are made up of more thaan one cell, with groups of cells differentiating to take on specialilzes functions. Unicellular organisms are made up of only one cell that perform all of the functions needed by the organism.
Multicellular organisms gave set if cells that specialize in reproduction which is usually sexual. During fertilization, when an egg and spoerm fuse, the resultant zygote is considered to be unicellular.
There is no excretory organs in a unicellular organisms. They remove their waste by simple diffusion from the body surface to the surrounding water. In multicellular organisms, excretion occurs by specialised organs called kidney.