"He lifted his eyes and saw: And behold! Three men were standing before him…" (Genesis 18:2)
When the angels visit Avraham, they’re called “men,” as we see in this verse and in verses18:16 and 18:22. However, shortly later, when the angels visit Lot, they’re suddenly called “angels,” as we see in 19:1. Why the switch? The Pardes Yosef -- a classic commentary on the Torah -- says that this teaches us the essential difference between the hospitality styles of Avraham Aveinu and Lot. Avraham truly excelled in Hachnasas Orchim (welcoming guests), and whomever he invited – even if they were ostensibly ordinary people (ie. “men”) – was treated unbelievably well. In this section of the Torah, we see that Avraham invited the three men into his house even though he suspected they were nothing more than idolatrous desert nomads (see the comment of Rashi to 18:4). However, when it came to Lot, his style of Hachnasas Orchim was quite different. He only invited people into his home if he thought were very chashuv (distinguished) and that their presence would add to his prestige (ie. “angels”). This is why Lot was quick to welcome the angels into his home!