Slack – Difference Between Starring and Pinning

According to slack:

Stars are a way to mark an item in Slack as important. You can star channels or direct messages to move them to the top of your left sidebar, or star messages so you can easily come back to them later.

Any items you star are marked only for you. To mark something as important for everyone on your team, try pinning the item to a channel.

And for pins:

Important messages and files can be pinned to the details pane in any channel or direct message, including group messages, for easy reference.

A message will appear in the channel or DM letting others know you’ve pinned an item. Your pinned item will be shown in the details pane, which you can open by clicking the pin icon.

Note: A maximum of 100 messages or files can be pinned to any one channel or direct message at a time.

SO, basically, starring is just for you, and pins are public/communal.
Pinning things also adds it to the channel details for easy access to everyone.