SCSH is a shell embedded in Scheme, i.e. it's a Scheme library that lets you easily create unix processes in Scheme. Schemesh is Scheme embedded in a shell, i.e. it's a shell that lets you easily call Scheme code.

For example, if you type:

    ls

in schemesh you will execute the "ls" command and get a directory listing, whereas in scsh you will get the value of a variable named "ls".

[UPDATE] Also, as cosmos0072 notes in a sibling comment, schemesh has shell-like features like line editing, autocompletion, searchable history, aliases, builtins, etc.