Your body is made of lots of systems, which are made up of several organs working together. The organs that make up the systems are made up of tissues, which are groups of cells, the things every single thing in the human body is made of.
Because of our lifestyle, we have certain features. For example, our eyes are on the front of our head, instead of the sides, because we have no reason to have to look for predators hunting us down. Gazelles, on the other hand, have lots of predators hunting them, so they have their eyes on the side of their head, for wider vision. We are also are adapted to earth. For instance, we have lungs that breath in earths atmosphere comfortably. So is aliens from other planets exist, they would probably be adapted to whatever planet their from. You see, aliens on an ocean planet would have gills instead of lungs to breathe. And aliens on hot planets would have darker skin, so that the sun doesn't burn them.