For example:
* Teacher: One who teaches.
* Writer: One who writes.
* Singer: One who sings.
* Worker: One who works.
There are other suffixes that can indicate "one who does" depending on the context, such as:
* -ist: (e.g., artist, scientist)
* -ant: (e.g., assistant, applicant)
* -or: (e.g., operator, conductor)
However, -er is the most common and versatile suffix used for this purpose.