Most helots presumably worked as farmers, something that was considered beneath the dignity of Spartan citizens. They owed whichever Spartiate had been assigned their plot of land a certain portion of whatever they farmed. Most of them probably lived out on that land rather than in the city. A minority of them worked in Spartiate houses as servants, guards, and grooms. The responsibility for clothing the Spartiate class fell almost entirely on them, since unlike citizen women elsewhere in Greece, Spartan women spent very little time on textile work.
Helots outnumbered Spartan citizens by around seven to one and unlike other slave populations, the number of helots expanded mostly through reproduction, meaning that their population could stay relatively stable without expansionist policies. Given this difference in numbers, it’s not really surprising that the Spartiate class lived in fear of a helot rebellion. Every year they declared “war” on the Messenians and during this time would actively hunt them down, especially the strongest among them.
There are also instances of Spartan men having children with helot women. The resulting offspring were termed mothakes* and were free but not citizens. Most remained in the lower levels of Spartan society, but a few rose to prominence as generals. Spartans may also have recruited some helot men as soldiers, especially in the early 4th century, typically rewarding them with freedom if they survived the campaign.**
The helots of Sparta remained a distinct and subject population, considered part of Spartan society but also considered property. When Epaminodas invaded the Peloponnese in 371 BCE, he freed the Messenian helots. The Laconian helots wouldn’t be freed for another hundred years or so when the reformer kings came to power.
*Probably. Ancient writers used this term to mean a couple of different, but similar, things. We have no written sources from Spartans on the topic. The closest we’ve got is Xenophon, an Athenian who lived in Sparta for much of his life.
**There’s some uncertainty regarding this claim. It may be that they recruited already freed helots to fight instead.
Pomeroy, Sarah B. Goddesses, Whores, Wives, and Slaves. New York: Schocken, 1995.
Pomeroy, Sarah B. Spartan Women. New York: Oxford University Press, 2002.
Talbert, Richard J. A. "The Role of Helots in the Class Struggle at Sparta." Historia: Zeitschrift für Alte Geschichte 38.1 (1989): 22-40.
Hunt, Peter. "Arming Slaves and Helots in Classical Greece." In Arming Slaves: From Classical Times to the Modern Age, edited by Christopher Leslie Brown and Phillip D. Morgan, 14-39. Yale University, 2006. [Note: May be found on Google Books here.
Helot - Livius.org