the song of purushahow are the four castes described in this veda
In the Vedic Hymn to Purusha, the world comes from the sacrifice of a giant being named Purusha. The gods make an altar for him and perform the sacrifice. The directions comes form his body, the sun and the moon from his eyes, the wind from his breath, etc. And the four classes (castes) come from his body.
Specifically, the Brahman class comes from his mouth, because the brahman priests chant the Vedas. The Kshatriyas come from his arms, perhaps as a symbol of their being the military class and nobility (ie. arms are for holding weapons). The Vaishyas, the merchant class, come from the thighs of Purusha. And the lowest class, the Shudra, the servants, come from his feet, symbolizing their low and "unclean" status.
Note also that there are two important themes here. One is the symbolism associating all the things and people that are created with the different parts of Purusha (ie. his breath is the wind). The other is that all things come from the same source, Purusha, indicating that everything is on some level unified.
Here's a copy of the hymn online: