What's the difference between `-` and `--` in shell commands

Broadly, it's up to individual programs to decide what their parameters look like, so anything I say will not be universal.

However, by common convention, a double-dash is used for longer parameter names, and single-dash is used for single-character names. To take an example from the Linux ls command:

    -d, --directory
          list directories themselves, not their contents

So you can use either the short or long forms in this case. Further, short parameters can often be combined... e.g. you can run ls -lrt instead of ls -l -r -t, which is convenient when typing in a shell.

In contrast, the longer names are often more descriptive, so while I'd rarely use them when running commands in a shell, I'd often do so when writing scripts (where long-term readability becomes a factor).

Again though, this is only convention, and not everyone follows it. Some tools don't both with a dash at all (e.g. tar xzf filename.tar.gz), and others will use a single dash even for longer names (e.g. all of the Java command line tools).