Serfs were a bit similar to slaves, but a slight bit more well off. While slaves were considered the property of their owners, serfs were not. And while they did not live the luxurious lifestyles of those they served, they still had their place, and had their limited rights (even if these rights may or may not have been actually enforced). Serfs were essentially attached to land, with feudal contracts. The owner of the land could not sell the serf to another and force him to go somewhere else, and if their lord sold the land they lived on to another, the serfs most often followed.

Serfdom was more of a contract. Serfs lived and worked the land that the Lords owned, and in return had the Lord’s protection and a place to live/work. Now a serf could not leave the land without his Lord’s permission, and could not sell the land, they still had a bit of freedom, and a day or two per week when they could work for themselves to raise money.

This most often began when one accumulated a very large amount of debt. He would then go to a lord, and get into a feudal contract of serfdom. This meant that the Lord would protect the man and slowly pay off his debt, in return for the man living on the land and working the fields for said lord. Then the man’s children became serfs to work off the debt, so on and so forth.

So unlike slaves, serfs had rights, and actually got something back from their work, even getting paid.

This actually became an issue for the Lords during the Black Plague, because most serfs died off, allowing the remaining serfs to demand more payment for their work, allowing the serfs more free time to hone crafts skills and move away from their lords, and into the cities for better paying jobs, but that is a whole another story.

Tldr: slaves were property and treated as such. Serfs were citizens in contract with their lords, receiving paychecks and actual compensation for their work, as well as having rights, but they were still attached to the land, and leaving serfdom was extremely difficult.