These are some of the greatest works of epic literature in the Western canon. Moreover, they are complex pieces of work with many twists and turns. Hence, any summary will be a bear sketch. So, I encourage you to read both works when time permits. With that said, here are a few points of summary.
First, Homer, the Greek poet is said to have written both. The Iliad is about the Trojan War, which started because the Trojan prince, Paris took Helen of Sparta from Menelaus. So, as revenge Menelaus got the leaders of Greece to fight the Trojan. However, things got complicated due to the conflict between Agamemnon and the greatest hero among the Greek, Achilles. Achilles, therefore, withdrew from battle, which caused the Trojan to gain the upper-hand. Achilles only went back into battle, because his good friend Patroclus was killed. Achilles then defeats many of the Trojans, and more importantly the great hero of the Trojans, Hector. This turns the tide. From this perspective, the Iliad is about the wrath of Achilles.
The Odyssey is about what happened after the Trojan war. Odysseus, one of the Greek heroes is on his way home back to Ithaca. However, his journey is fraught with twists and turns. In a word, things get very complicated. For instance, he has to face many dangers - the sirens, Polyphemus, Circe, Calypso, and many more. When he finally gets back, he realizes that there are many suitors that have taken over his home. So, he needs to fight them and he does. In short, this work is about his journey and homecoming.