The two fields of study are: Morphology and anatomy
I'll give you an example of it, imagine that you found a mysterious insect in the forest, you don't know anything about it, you take it with you and give it to your biology teacher. The first thing that he is going to take a look at is at its anatomy, how the insect looks like, how many paws it has, wings, colors and etc.
After that, the teacher will probably ask you a few questions about the place you took the insect from, how was it, was it deep, lots of trees around, water near it, any fruit trees nearby, and by that, the teacher will analyze how this animal probably live, just by its habits with your information, if you're not sure about it, he'll need to check it out about how its body works, using again morphology and anatomy, and by doing that, is how you classify an organism.