The s command is similar to the c command in that it puts you in INSERT mode, but it only really does two things that the lower c does.

With s you can also use it visual mode to highlight a block and the substitutions or deletions will get place in a register that you can later grab from.

The lowercase s command on it's own will delete the character under the cursor and put in insert. This command is an alias for cl,

The Uppercase S will delete a whole line. This command is also an alias for the cc command.