The gods and goddesses portrayed in Homer's Iliad are anthropomorphic, meaning that they take forms like those of human beings. Even though they are immortal and have great powers, the gods have human characteristics and passions and are capable of interbreeding with humans. Most of the heroes of the Iliad can trace their lineage back to a god having mated with a human. Helen of Troy, for example, is a daughter of Zeus and the mortal Leda.
The gods tend to intervene quite directly in the events of the epic, supporting one or the other side of the war and helping their favorite heroes in key battles. The gods demand worship, sacrifices, and obedience from humans, and in return the gods give them various favors in a system often called "do ut des" (I give that you might give).
The origin of the Trojan War in the story is the Judgment of Paris, which involves three goddesses, Hera, Athena, and Aphrodite, competing to be awarded a golden apple inscribed "to the fairest" that the goddess Eris (strife) had thrown into the crowd at the wedding of Achilles's parents. As a consequence of this contest, Aphrodite helps Paris obtain Helen.
Aphrodite, Apollo, and Poseidon support the Trojans, and Athena and Hera support the Greeks. They intervened both directly and indirectly in the war.