There are two distinct words “nafs” and “rooh”, which are both translated as “soul” in English. The word rooh is the same as the English word for soul. Nafs, however, is slightly different. The word nafs is defined as “a feature of the soul. This feature continuously directs people to committing sin.”★ The desire to commit evil acts, such as lying and cheating, is therefore a result of the nafs.
When a person follows the demands of his/her nafs, it grows stronger and demands more. Fighting against its wishes and worshiping more often, on the other hand, weakens the nafs and diminishes its impact on the person’s life. That being said, nafs can also be used to define “the self” as a whole instead of only the evil portion of it.
Source: my blog