A share room gives you a place to share a folder full of files. They do not need spideroak. Share rooms also allow you to password protect the entire folder. I've written about it here.
Share rooms allow you to have a key that even Spideroak does not possess, so the files are as secure as your password.
Shared Files
Shared files are the very secure but not quite perfectly secure way to share a single file. The huge advantage is that it is so easy. The files are encrypted on their server and they automatically delete in three days.
However, since Spideroak does have the key so they can decrypt the file for the person receiving it, there is a little less perfect security than a share room with an extra password.
To put the risk in perspective, I'll be happy if my CPA sends me my taxes using the Spideroak File share. I see no reason for more security.
How to do it
Backup the file you want to share.
Open Spideroak to the Manage tab
Find the file you want to share in your backup
Click the Link button in the upper right.
Wait a bit while Spideroak creates a secret key and uses it to encrypt and save your newly encrypted file to random location on their servers.
Large files will take a few minutes to encrypt and save.
Right click the newly created link and select all. Then copy it.
Paste it into an email to the person you want to share with
When your client or friend receives the email, they can just click on the link in it to open or save the file you are sharing.