/bin/sh is where you will find the ubiquitous and somewhat spartan Bourne shell, written by Steven Bourne.
Most shell scripts call on the sh shell because of its very ubiquity. It's been around since the 70's and it's not going to go away anytime soon. This makes shell scripts that use the bourne shell very portable -- sh will very likely be on literally every unixlike machine you ever encounter, but if you code a shell script that uses special features specific to other shells, you'll soon enough find a system that doesn't have them. (Jimbo recommends the following: "If it's an incredibly simple task, write a shell script in sh. If it's not, forget the shell scripting and write a perl script instead." Ancient Unix wisdom says that if you can't do it in sh, do it in C.)