sh is a shell, a program that provides a command-line interface to your computer. It can be used either interactively, or as an interpreter for script files, such as the famous periodic maintenance scripts. Some such script is likely running in this case, and may be stuck for some reason.
You should be able to find out what script that is by using Inspect on the
sh processes in Activity Monitor and looking at the Open Files and Ports tab. A file listed there with the .sh extension is a shell script. Its name will hopefully give you a clue as to its purpose; you can also open it as text and inspect it.