Summary
The 105th Drop Jet Platoon was a UNSC unit of marines. In 2535, the 105th was dispatched to the planet Jericho VII where they fought Covenant ground troops. However a force of some 1,000 strong Grunts overwhelmed the 105th and slaughtered the entire unit.
Later, the Spartans avenged the 105th by slaughtering the Grunts responsible for routing the 105th.
It should be noted that there is a 105th Orbital Drop Shock Trooper Division as well. Perhaps this unit was an early band of ODSTs?