Question

Why do we see so many different kinds of organisms around and how do we recognize them ?

Answer 1

Reproduction process has inbuilt tendency for variation .error in DNA copying during reproduction cause variations and result into variety of organisms.
evolution leads to formation of two different species which is caused by genetic drift, natural selection,geographical isolation and severe change in DNA of germ cell. Its is usually caused when there is geographical isolation in above case and result into species which cannot interbreed.

One common way to identify plants and animals is to use a dichotomous key. You start off by looking at the big, obvious features of the organism, and then look at more detailed and particular features to narrow it down to one species. Two different species of animals that look exactly the same may be different in behavior.

Identification of Bacteria, Eukarya, Archaea, and viruses (if you consider them to be organisms) can be much trickier and can even require DNA analysis in many cases.