what is the main difference between a protozoa and a bacterial cell?

Protozoa are unicellular organisms with a discrete nucleus and discrete organelles (the things that make the cell work, like mitochondria - the energy converting components). They belong to the eukaryotes (as do we).

Bacteria have no discrete nucleus or organelles (although there may be nucleic matter and other things that make the bacteria "function"). They belong to the prokaryotes.

A recent estimate (in "Nature", I think) reckons that the biomass of all the bacteria and archaeans (similar forms of life to bacteria) on the planet is probably around 80 times the mass of all other living things on the Earth put together.

There are loads of the little blighters!

One, Deinococcus radiodurans has been detected that can survive a thousand times the level of ionising radiation that would kill any multicellular animal.

Some are tough little blighters!