Why does 'ps -ef' show multiple "kworker" entries?

[joe at actionline.com]

What is "kworker" and why does 'ps -ef' show multiple entries?

$ ps -ef | fgrep kworker
root        57     2  0 Jan24 ?        00:00:02 [kworker/u:3]
root        58     2  0 Jan24 ?        00:00:00 [kworker/u:4]
root      2037     2  0 10:03 ?        00:00:00 [kworker/3:2]
root      2267     2  0 10:49 ?        00:00:00 [kworker/0:2]
root      2313     2  0 10:54 ?        00:00:00 [kworker/0:0]
root     23855     2  0 Mar25 ?        00:00:03 [kworker/2:1]
root     27073     2  0 Mar25 ?        00:00:00 [kworker/2:2]
root     30049     2  0 Mar25 ?        00:00:00 [kworker/1:1]
root     30109     2  0 Mar25 ?        00:00:00 [kworker/1:0]
root     32039     2  0 00:38 ?        00:00:16 [kworker/3:0]